An improved pre-processing method for smooth particle hydrodynamics is proposed, which can rapidly distribute the uniform particles for the fluid field with an arbitrary boundary. The improved pre-processing method is obtained by improving the algorithm proposed by Colagrossi et al., whose method can distribute the uniform particles in a simple shape. However, when the ratio of the smoothing length to the initial particle space is bigger than unity, the algorithm proposed by Colagrossi et al. has a numerical oscillation and a slow convergence rate; and the fluid particles may penetrate the solid boundary. For solving the problems, the XSPH model is added to improve the algorithm stability, and the boundary force is exerted on the fluid particles near the solid boundary to prevent fluid particles from penetrating wall surface. Verified by showing some exmples, the improved pre-processing method can rapidly distribute the more uniform particles for the fluid field with an arbitrary boundary and prevent fluid particles from penetrating wall surface.