In this paper, through the research on the longitudinal and transverse depth control of the small amphibious underwater vehicle, through the mechanical structure design of ROV system and the structure distribution of thruster, through the generalized multi degree of freedom equation, under multiple constraints, the bi-directional depth control model is established. Through the UFD fluid analysis in fluent, the hydrodynamic coefficients in different attitude motion control states are obtained Through the research of PID control and fuzzy PID control theory, the PID controller and fuzzy PID controller of depth control is built in Simulink Library of Mtalab. The control effect of PID controller and fuzzy PID controller is compared. The simulation results show that the fuzzy PID control is more effective than PID control Fuzzy PID has good stability and anti-interference ability for depth control and angle control in vertical and horizontal control .