Abstract: The problem of high-speed transmedium water entry widely exists in the fields such as ocean engineering, aerospace, and more. The mechanism of multiphase flow and fluid structure interaction in high-speed cross medium water entry is a research hotspot in this field, which is of great significance for load prediction, ballistic stability evaluation, structural strength verification, and safety design of transmedium weapons and spacecraft. This paper focuses on the fundamental key mechanical issues involved in high-speed transmedium water entry, focusing on the research status of multiphase flow and cavity evolution, impact load and load reduction methods, motion stability and fluid structure interaction response, and numerical methods of fluid structure interaction. Suggestions and prospects are proposed for the existing problems, aiming to provide fundamental references for related research and design of high-speed transmedium water entry.