Abstract: Aim To study the unstability of Chaplygin system's equilibrium state manifold under the random perturbation. Methods Firstly, the stochastic differential equations of Chaplygin system was established; then, the first and second linearization moment equations corresponding to the system were given, at last, the moment unstability of the stochastic system according to the theory of Liapunov was judged. Results and Conclusion The criterion for juaging the unstability of stochastic Chaplygin system was obtained and one example was given to illustrate the application of the criterion.