The invention provides a method for realizing focal spot reconstruction based on optimal matching point quick search of a steepest descent method, so that measurement of laser focal spots in a high dynamic range can be realized, and the reconstruction precision and efficiency of far-field focal spots are improved. The method comprises the following steps: respectively converting an image cutzb' and a side lobe cut image cutpb obtained by deducting a circle with the size equal to that of a schlieren ball from a main lobe cut image into two column vectors, and calculating SAM values of the two vectors; Calculating the gradient of an eight neighborhood of the initial search position, and searching along the direction with the maximum gradient in the eight neighborhoods by using a steepest descent method; After all the SAM values meeting the condition points are searched, taking the position corresponding to the left upper corner of the main lobe shear image as an optimal matching point when the SAM values are minimum; And by taking the optimal matching point as a reference, reconstructing the focal spot by using the main lobe and the side lobe, and fusing splicing regions of the combined image by using a weighted average method.