Abstract: Simulation of severe accident in nuclear reactor involves multiphase flow phenomena containing free surface or component interfaces. The moving particle semi-implicit (MPS) method waiving the topological mesh has been extensively employed for multiphase simulation. However, the uniform-size particle used in the traditional MPS method cannot provide high enough spatial resolution in the region of interest, e.g., near the phase interfaces, since utilization of uniform-size fine particle significantly raises the computation cost. Meanwhile, a large number of wall particles are utilized to model the wall boundary, which remarkably increases the computation resource. In order to improve the computation efficiency and accuracy, the particle splitting algorithm is implemented along with the polygon wall boundary. Utilization of particle splitting in the specified region refines simulation of flow with complex free surfaces. The polygon wall model eliminates the wall particle. In the model, the wall effect is modelled based on a flat wall scenario where the contribution of the wall to the particle number density is estimated as a function of wall distance. The zero-pressure gradient at the wall is realized with the virtual mirror particles about the wall. In order to facilitate the implementation of particle splitting algorithm and polygon wall model, several basic models of the MPS method were modified, including particle effective radius, particle number density, gradient operator model, Laplace operator model, and free surface identification model. For algorithm verification, the static pressure of water column was simulated with the improved MPS method. The discrepancy between the pressure at the monitoring point and the theoretical value is 6.25%. Then the baffle-free dam-break flow was simulated. The simulation is in good agreement with the experiment, which indicates that the particle splitting algorithm has little effect on the flow pattern. In the simulation of dam-break with baffles, the free surface motion can be replicated with the modified MPS method. The modified method predicts appreciably larger pressure fluctuation range than the traditional MPS method, which should be improved in the future. The splitting model can reduce the computation time by about 55%, while the polygon wall can reduce the computation time by around 62.5%. Combining both of the models, the computation time can be reduced by over 80%. Hence, the particle splitting model together with polygon wall boundary can improve local resolution and computational efficiency in the same time, which solidates a foundation for the particle merging calculation, three-dimensional calculation, and phase change heat transfer calculation.