Heaving motion is an indispensably function for underwater robots. A bionic bladder system which can realize both heaving and pitching control was designed inspired by biologic fishpsilas bladder. It was mainly composed of a cylinder and a piston. The dynamic models of the bionic bladder system for heaving and pitching motion were established and based on which a dual-velocity control system was designed. The two control channels became decoupling by setting different control cycles. Simulations with the real design parameters were performed. Results indicated that a good control performance can be achieved by the bionic bladder system with proper control parameters.