In this paper，using diffusion model and {c，h，α} three parameters，we describe nuclear deformation and set up a three-dimension nonlinear Langevin equation. The nuclear deformation motion is simulated with M-C method. The fission fragment mass distributions of highly excited nuclei ¹⁹⁸Hg, ^21OPo and¹⁸⁵Re are calculated，the mean mass，root mean square of the mass variance and the most probable values of the fission fragments are given. The calculated results are in agreement with experimental results qualitatively.