Through the underwater ignition of rocket engine , underwater vehicles could get more comprehensive performance advantages and flexible use capability. For underwater ignition of large rocket engines, direct experimental test requires large funds and a long cycle. Scale test technology is an effective means to study the performance laws. This paper analyzes the underwater ignition process of rocket engine. Through studying control equations and scaling criterion,the scaling similarity laws and the main parameter similarity relationships of rocket engines during underwater ignition are obtained,which provides a theoretical support for the scale test technology of a certain rocket engine. On this basis,the scale test of engines during underwater ignition is carried out,and the thrust characteristics of engines under different water depths are obtained.