An optimum scanning beam searching method of linear array push-broom image based on object space projection geometric constraint is provided, which divides CCD linear array sensor into sections and indicates each section by a straight line; when the CCD detector scan imaging, a scan imaging center projection plane is formed between the CCD detector and the scan imaging center; the six affine transformation indexed from the average height projection plane coordinates of the linear array push-broom image object space to the image scanning coordinates are evaluated; the evaluated value of neighboring scanning line central projection plane space distance of image is gotten; evaluate the imaging position of object on image; iterative computation of scanning line object space searching is conducted; two scan line central projection planes nearest to the object point are positioned, the exact solution of the optimum scanning line object space is counted through object space interpolation. The invention is simple and easy to realize, which greatly reduces the counting amount of optimum scanning beam searching, thus being able to improve the efficiency of coordinate back projection counting of linear array push-broom image. The invention is particularly applicable processing the linear array push-broom mass images of high resolution.