This paper studies the mechanical interface design technology of powerplant system for civil aircraft. Three different types of powerplant system mechanical interface and associated characteristics were presented. The paper concludes the main design work of mechanical interface design, and proposes general process of mechanical interface design of powerplant system. The detailed design work and process are presented with taking the hydraulic pump mechanical interface design as example. The research supported the installation integration design of powerplant system for civil aircraft and the accumulated technology.