Abstract: Image segmentation is a key part in image processing field. Based on the entropic thresholding segmentation methods, a novel algorithm based on particle swarm optimization is presented in this paper. We also analyze the convergence of the new algorithm based on Bayes’s theorem and stochastic transform process. In experiments, benchmark image and SAR image are selected, and the algorithm runs ten times independently and the running time is selected as the evaluation of the algorithm complexity. It shows that the algorithm presented in this paper can find better solutions with much little complexity. Experiments results show that this method is feasible and effective