1. An automatic recognition method of foot point of continental slope based on topography grid, comprising the following steps of: (1) grid cutting: using a straight line f(x, y) to cut a topography grid model z i,j =Grid (i, j) to generate a two-dimensional topography section line, where origin coordinate and end point coordinate of the straight line f(x, y) are respectively O (x1,y1) and E (x2,y2) ; x i,j and y i,j are values of the x-coordinate and the y-coordinate of the grid point in row i and column j; and z i,j is a water depth value of the grid model in row i and column j; a slope of the straight line f(x, y) is that k=(x1−x2)/(y1−y2), when y1=y2, the slope is expressed as: k=(y1−y2)/(x1−x2): (2) first derivation: carrying out first derivation on the two-dimensional topography section, and obtaining a point set of the coordinates and water depth value of the point of intersection between the straight line f(x, y) and the grid model Grid (i, j) through an intersection operation between the straight line f(x, y) and the grid model based on the grid model of the submarine topography Grid (i, j); generating an original data point set G 0 ={g i } through step (1) and step (2), where each data point comprises distance, water depth, slope and second derivative value; where the coordinates and water depth value dep(x, y) of the point of intersection between the straight line f(x, y) and the grid model Grid (i, j) are calculated according to the following situations: (a) the point of intersection is located on the grid point Grid (i, j): directly returning the distance between the coordinates (x i,j , y i,j ) and the water depth z i,j of the grid point, and the origin O(x 0 , y 0 ): dis(x, y)=√{square root over ((x−x 0 ) 2 +(y−y 0 ) 2 )}{square root over ((x−x 0 ) 2 +(y−y 0 ) 2 )}; (b) the point of intersection is located on the row: when the point of intersection is located on row i and column j to j+1, the distance calculation is the same as step (a); the y-coordinate of the point of intersection: y=y i,j , when y1=y2, y=y1; the x-coordinate of the point of intersection: x=x1+(y−y1)×k, when y1=y2, x=x i,j ; the water depth value of the point of intersection: dep(x, y)=z i,j +(x−x i,j )×(z i,j+1 −z i,j )÷(x i,j+1 −x i,j ); (c) the point of intersection is located on the column: when the point of intersection is located on column j and row i to i+1, the distance calculation is the same as step (a); the x-coordinate of the point of intersection: x=x i,j ; the y-coordinate of the point of intersection: y=y1+(x−x1)×k, when y1=y2, y=y1; the water depth value of the point of intersection: dep(x, y)=z i,j +(y−y i,j )×(z i,j+1 −z i,j )÷(y i,j+1 −y i,j ) (d) the point of intersection is located in the grid; when the point of intersection is located on row i to i+1 and column j to j+1, the distance calculation is the same as step (a); the water depth value dep(x, y) of the point of intersection may be calculated according to an inverse distance square weighting method; the point of intersection is surrounded by four adjacent grid points, then the water depth value of the point is: dep ( x , y ) = ∑ i = 1 i = 4 w i z i ∑ i = 1 i = 4 w i , wherein w i = 1 d i 2. z i , w i and d i are respectively the water depth value, the calculated weighted value and the distance value from the point of intersection of the four grid points; the value of the y-coordinate of the foregoing point of intersection is: O (z1,y1) or E (x2,y2) ; (3) first simplification: obtaining an extreme point of the second derivative section line and taking the extreme point as an inflection point to generate a new firstly simplified topography section line; generating a data point set G 1 ={g i } after the first simplification through step (3); (4) second simplification: using a D-P algorithm to carry out operation on the extreme section and reserve the data points complying with the D-P algorithm, thus obtaining a D-P topography section after the second simplification; (5) second derivation: adopting the method in step (2) to carry out second derivation on the D-P topography section to obtain a new slope section line and a new second derivative section line based on the D-P topography section; generating a data point set G 2 ={g i } after second simplification and derivation through step (4) and step (5); (6) concave hull elimination: using a topography and slope judgment method to recognize and eliminate the concave hull topography in the D-P section; and generating a data point set G 3 ={g i } after eliminating the concave hull topography through step (6); (7) Integrated judgment: using slope, water depth, second derivative, concavity and convexity, features, continuity and segmentation judgment methods based on the D-P topography, slope and second derivative section to automatically recognize foot point of the continental slope in the topography section and automatically mark the foot point of the continental slope in the two-dimensional section, where the itemized judgment steps are as follows: (a) slope method: counting the mean slope values ms i in the point set G 3 ={g i } by regions, and respectively obtaining the mean slope g 1 in the shelf and sea basin region and the mean slope g 2 in the continental slope region; traversing the point set G 3 ={g i }, where the points having a mean slope ms i ∈| g 1 −Δg, g 1 +Δg| are in the shelf or sea basin region, and the points having a mean slope ms i ∈| g 2 −Δg, g 2 +Δg| are in the continental slope region; (b) water depth method: traversing the point set G 3 ={g i } to carry out depth sorting on the data point of the mean slope ms i ∈| g 1 −Δg, g 1 +Δg|, and obtaining the mean water depth value d 1 of the shelf and the mean water depth value d 2 of the sea basin; traversing the point set G 3 ={g i } again, where the water depth value dep i ∈| d 1 −Δd, d 1 +Δd| is defined as the shelf, and the water depth value dep i ∈| d 2 −Δd, d 2 +Δd| is defined as the sea basin; (c) second derivative: the foot point of the continental slope FOS is the point of the subsea having the maximum change of slope in the region from the continental slope to the sea basin, which is namely the second derivative extreme point; (d) convex hull feature: the foot point of the continental slope FOS is located at the turning position from the continental slope to the sea basin; therefore, the topography has convex hull feature, which is represented by the feature that the second derivative value is a positive value data point; (e) segmentation method: traversing the point set G 3 ={g i }, where the data point complying with the up slope |us i |∈| g 2 −Δg, g 2 +Δg| and the down slope |ds i |∈ g 1 −Δg, g 1 +Δg| is preliminarily judged as the foot point of the continental slope FOS; (f) continuity method: traversing the point set G 3 ={g i }, recording the growth distance bd i of each point tracked towards the original point pd i and the growth distance of each point tracked towards the tail point; traversing the point set G 3 ={g i } again to compare the bd i value and the pd i value of each point, where the point having the furthest distance is the foot point of the continental slope FOS; integrally apply steps (a) to (f), where the data point complying with the conditions of (c)˜(f) at the same time is the foot point of the continental slope FOS.