Abstract: As a meshless method,the Moving Particle Semi-Implicit Method is used to calculate the in-compressible fluid flow. The fluid is represented by a set of moving particles with full Lagrange description,and the differential operators in governing equations are discretized to the interactions of particles. With this method,two-dimensional Rayleigh-Taylor Instability （RTI） is simulated. It is demonstrated that,when the amplitude is much smaller than the wave,the perturbation of interface grows exponentially at early stage of the instability,which is consistent with the linear theory. During non-linear stage,the characteristics of bub-bles and strikes obtained in the calculation agrees with the previous experiment to some extent.