1. A method for measuring lens distortion, comprising: providing a test card having a dot matrix pattern of K×N dots, wherein the K and the N are both natural numbers, and the K is equal or unequal to the N; obtaining a distorted image of the test card after being distorted by a lens; establishing a planar coordinate system for the distorted image by using a dot at an upper left corner of the distorted image as a coordinate origin, a rightward direction from the origin as a positive direction of axis X, and a downward direction from the origin as a positive direction of axis Y; positioning a center dot and all non-center dots of the distorted image by scanning and searching, and determining coordinate values of the center dot and all the non-center dots in the planar coordinate system; and calculating a distortion amount of the distorted image by using the coordinate values of the center dot and all the non-center dots, thereby obtaining a distortion amount of the lens; wherein the positioning a center dot of the distorted image by scanning and searching, and determining coordinate values of the center dot in the planar coordinate system comprises: setting a square searching area centered at a coordinate of a center of the distorted image, a half of a side length of the square searching area being larger than or equal to two times of a radius of the center dot; setting a square scanning area centered at a dot at an upper left corner of the square searching area, a half of whose side length is equal to the radius of the center dot; starting from a center of the scanning area to scan the scanning area, and calculating an average value of all pixel points in the scanning area; in the searching area, sequently moving the center of the scanning area until the searching area is completely scanned; comparing the average values of the pixel points obtained in the scanning area each time with one another, and determining a center of a scanning area where a minimum average value or a maximum average value among all the average values of the pixel points is located as a center of the center dot; and determining in turn a coordinate value of the center of the center dot, and determining the coordinate value of the center of the center dot as the coordinate value of the center dot.