A theoretical model is made to analyse the effects of finite-size particle，charge nearest grid sharing and time difference on the electrostatic dispersion relation of plasma. A modified dispersion relation is derived. The computer experimental scheme to simulate the plasma particle is designed. The simulation result shows the effects of finite-size particle and time difference on dispersion relation. Finally，completely different simulation results of plasma soliton with different particle size are presented. It indicates that these effects may change the basic physical phenomena of the simulated results in some circumstances.