Material erosion under a multiphase flow is a very complex process influenced by many parameters. Understanding physical mechanisms and establishing governing laws for predicting the erosion rate are of great importance to alleviate or even avoid erosion damage in engineering applications. In this paper, we perform a combined numerical and experimental study to understand how the evolution of material surface induced by erosion can inversely affect the multiphase flow characteristics and erosion mechanisms on the surface. A water-sand erosion test-rig system is used to obtain the surface profiles, erosion rates and surface patterns of stainless steel under a water-sand multiphase flow. A multiphase flow model and an erosion model are combined to obtain the flow profile, erosion rate and erosion pattern. To gain insights into the multiphase flow and erosion mechanism changes due to the surface evolution induced by the erosion process, we take the surface profiles obtained from our testing samples at different stages of experiments to create geometry models for our numerical simulations. The numerical results are in good agreement with experimental results for erosion rate and erosion pattern as well as erosion mechanism. Through systematic numerical simulations, we clearly reveal the detailed changes in multiphase flow characteristics and erosion mechanisms arising from the surface evolution. The present work shows that the erosion process is very sensitive to the change of the test sample surface resulting from the change in erosion mechanism, highlighting the need to include the surface evolution in erosion modeling.