The deep profiling float is one of the most effective instruments for acquiring long-term and quasi-real-time subsurface ocean data in a wide range. Due to the change of sea water density and the overall compression deformation of the float, the buoyancy level of the float will change during the process of descent and ascent, and it shall be regulated with deformation of the volume of the oil bladder driven by the buoyancy control module. With the increase of water depth, the required amount of buoyancy regulation gradually increases, and the power consumption also increases. In this paper, a passive buoyancy compensation device is designed, which makes use of the compressible characteristics of the medium to offset the buoyancy loss, reduce the amount of buoyancy regulation and reduce the power consumption of the buoyancy control module. The effectiveness of the module is verified by offshore application, and the problems existing in the module and the direction of future improvement are analyzed.