1. A three-dimensional measurement method based on wavelet transform, comprising the following steps: Step 1 : projecting a monochrome fringe pattern onto a surface of an object to be measured, taking a picture of the object surface with a charge-coupled device (CCD), to obtain a deformed fringe pattern g(x,y) with height c and width r: 
 g ( x,y )= A ( x,y )+ B ( x,y )cos [2π f 0 x +φ( x,y )] wherein, A(x,y) is a background light density distribution, B(x,y) is a reflectivity of the object surface, f 0 is a frequency of the sinusoidal fringes, φ(x,y) is a relative phase distribution map to be solved, and (x,y) represents two-dimensional coordinates of the deformed fringe pattern; Step 2 : performing wavelet transform for the deformed fringe pattern line by line, to obtain a map of relative phase distribution of the deformed fringe pattern, through the following steps: Step 2. 1 : treating y as a constant, and processing line Y of the deformed fringe pattern g (x,y) by means of one-dimensional continuous wavelet transform, as follows: 
 W ( a 1 ,b )=∫ −∞ +∞ g ( x,y ) M a,b *( x ) dx where, a is a scale factor, whose value is in the range of 10-50, and is taken at an interval of 0.2; b is a shift factor, which value is in the range of 1 to the width r of the fringe pattern, and is taken at an interval of 1, in unit of pixel; W (a 1 ,b) is a two-dimensional complex matrix in 200 lines and r columns; a 1 is the line label of the elements in matrix W (a 1 ,b) which is referred to as the wavelet transform matrix of line y, M a , b ⁡ ( x ) = 1 a ⁢ M ⁡ ( x - b a ) , M a,b *(x) is a conjugate function of M a,b (x), M(x) is wavelet function expressed as follows: M ⁡ ( x ) = 1 ( f b 2 ⁢ π ) 1 / 4 ⁢ exp ⁡ ( 2 ⁢ πⅈ ⁢ ⁢ f c ⁢ x ) ⁢ exp ⁡ ( - x 2 2 ⁢ ⁢ f b 2 ) wherein, f b is a bandwidth of the wavelet function, f c is a center frequency of the wavelet function, and i is a complex unit, Step 2. 2 : calculating an optimal scale factor distribution map a r (x,y) and relative phase distribution map φ(x,y) of the fringe pattern, wherein, the method for calculating values of a r (x,y) and φ(x,y) at coordinates (x,y) is as follows: calculating the modular matrix A(a 1 ,b) and angular matrix φ(a 1 ,b) corresponding to W(a 1 ,b) searching for the maximum element in column x of matrix A(a 1 ,b), and calculating line label a max of an element in matrix A(a 1 ,b); then, a rx =10+0.2×a max , where, a rx is a value of the optimal scale factor distribution map a r (x,y) of the fringe pattern at coordinates (x,y), a value of the element with line label a max in column x in matrix φ(a,b) is a value of the relative phase distribution map φ(x,y) of the fringe pattern at coordinates (x,y), traversing all coordinate points in the fringe pattern, to obtain the optimal scale factor distribution map a r (x,y) and relative phase distribution map φ(x,y) of the fringe pattern, Step 3 : creating a quality map Q(x,y), Step 3. 1 : performing wavelet transform for a one-dimension sinusoidal signals at frequency f 0 : 
 W 1 ( a 1 ,b )=∫ −∞ +∞ cos(2π f 0 x ) M* a,b ( x ) dx calculating modular matrix A 1 (a 1 ,b) of a two-dimensional complex matrix W 1 (a 1 ,b), searching for a maximum element in each column, recording a line label of the maximum element, calculating an average value ā of these line labels, and calculating an optimal scale factor a r1 : 
 a r1 =10+0.2× a Step 3. 2 : calculating a quality map Q(x,y): 
 Q ( x,y )=| a r ( x,y )− a r1 | Step 4 : dividing the relative phase distribution map φ(x,y) into two parts, according to the quality map Q(x,y); Step 4. 1 : creating a matrix D with value=0 in height c and width r, Step 4. 2 : traversing all points in the quality map Q(x,y), and calculating a value of element g that appears most frequently in the quality map Q(x,y), to obtain a threshold T=1.05×Q 1 , Step 4. 3 : traversing all points in the quality map Q(x,y), and setting a corresponding element in the matrix D to 1 when the value in Q(x,y) is greater than the threshold T, Step 5 : unwrapping a phase at points in the relative phase distribution map φ(x,y) where a corresponding element in matrix D is equal to 0, Step 5. 1 : creating a matrix S with value=0 in height c and width r, Step 5. 2 : choosing a point within a range of 20×20 around a center pixel of the relative phase distribution map φ(x,y) where the corresponding element in matrix D is equal to 0, as a start point for unwrapping, taking an absolute phase value of a start point as a value of that point in the relative phase distribution map φ(x,y), unwrapping the phases of the points towards both sides in a line direction where the corresponding element in matrix D is equal to 0, and, whenever a pixel point is unwrapped, setting the element in matrix S corresponding to the pixel point to 1; in the unwrapping process, if a point is encountered where its previous point in a same line has not been processed yet, taking a point neighboring that point in a previous or next line as a previous point and unwrapping the point; repeating the process for each line, wherein, at each point, a specific unwrapping process is as follows: φ unwrp = φ wrp + 2 ⁢ π ⁢ ground ⁢ ⁢ ( φ unwrp ⁢ ⁢ 1 - φ wrp 2 ⁢ π ) wherein, φ unwrp1 is a solved absolute phase value of a previous point, φ wrp is the relative phase value of a current point, which is a value of the current point in the relative phase distribution map φ(x,y), φ unwrp is an obtained absolute phase value of the current point, and round is an integral function, Step 6 : unwrapping the phase at points in the relative phase distribution map φ(x,y) where a corresponding element in matrix S is equal to 0, Step 6. 1 : marking connected domains where elements are equal to 0 in matrix S, and treating each connected domain as follows: Step 6. 2 : choosing any boundary point of a connected domain as a start point, and pushing the point into a stack that is empty at present, Step 6. 3 : searching for points in a points neighboring the start point where the corresponding element in matrix S is equal to 0; jumping to step 6. 5 if no such point is found in neighboring points; for each of the points found, unwrapping the phase at the point, setting the corresponding element in matrix S to 1, and storing the point in the stack, Step 6. 4 : sorting the points in the stack by their values at corresponding positions in the quality map Q(x,y) of the fringe pattern, with the point with the highest quality value placed at a top of the stack, Step 6. 5 : taking out a point at a top of the stack as a start point, and judging whether the stack is empty; terminating the unwrapping process for the connected domain if the stack is empty; otherwise jumping to step 6. 3 , Step 7 : obtaining the absolute phase map φ(x,y) of the fringe pattern through step 5 and step 6 , and ultimately obtaining three dimensional information of the measured object by means of a classic phase-to-height conversion formula for grating projection and then obtaining and displaying a point cloud image that represents the three-dimensional information of the measured object.