The present disclosure provides a method and a system for processing an image. The method includes: an image pre-processing step, including analyzing an image input to be processed so as to obtain a first image; and a spot detecting step, including analyzing the first image so as to compute a spot determining threshold; analyzing the first image so as to acquire a candidate pixel spot, and judging whether the candidate pixel spot is the spot according to the spot determining threshold; computing a sub-pixel center coordinate of the spot and an intensity value of the sub-pixel center coordinate if yes; and discarding the candidate pixel spot if no. According to the method, the image is denoised in the image pre-processing step, such that a calculation in the spot detecting step may be reduced. In addition, the accuracy of determining the image spot may be improved.