Aiming at the problem of bionic underwater vehicle’s research using fish swimming locomotion mechanism,from the point of view of image processing, fish’s soma moving model was established based on dynamic image sequences. First, attain fish’s contour by edge detecting, and present a method for soma extraction based on distance transform. Second, fit soma’s envelope and translational equation to attain parameter of slender body theory. Finally,establish fish’s soma moving model. The experimental results show that our model can describe fish swimming curve deformation under natural state more accurately and practically.