Three-dimensional simulations of particles coarsening in a solid-liquid two-phase system are investigated using the multiphase-field model. The evolution of the interface shape distribution during coarsening is analyzed. And the influences of the volume fraction on the interface shape distribution and coarsening rate are studied under different coalescence conditions. The simulation results show that the influence of volume fraction on the change of coarsening rate is delayed when there exists coalescence between solid particles under high volume fraction. Moreover, with the evolution of coarsening, proportion of the hyperboloid with high curvature decreases and the proportion of ellipsoid with low curvature increases. No matter whether the coalescence between particles occurs, the interface shape distribution has self-similarity after a period of time of evolution. But it will take a longer time for the system to reach the steady state with the increasing of volume fraction.