Abstract: The moving particle semi-implicit method was developed to simulate incompressible fluid using a meshless method. There was a big problem that the space distribution and the time variation of pressure oscillate drastically in the MPS method. In order to investigate the pressure oscillation in the MPS method, the simplified one-dimensional model was developed. The mechanism of pressure oscillation in the MPS method was illustrated by the movement and the relative position between the center particle and its neighbor particles. The collision model was employed in the simulation and the relative position between particles was controlled by choosing different collision parameters. The classical dam break problem was simulated. With the increase of collision parameter, the fluctuation of the deviation of particle number density decreased. According, the amplitude of pressure oscillation was suppressed. Two different kernel functions were also employed to investigate the pressure oscillations. The results showed that the gauss kernel function improved the stabilization of pressure calculation. It was the reason that the same movement of particles lead to less deviation of particle number density when the gauss kernel function was used. And the randomness of particles motion led to random fluctuation in the deviation of particle number density. As a result, the pressure fluctuation in MPS method occurred and it was difficult to be eliminated.