Buoyancy regulating device can be used in unmanned underwater vehicles to realize the energy-saving rising，diving or suspending in the water.In this study，a buoyancy-regulating device with lager volume and high precision was designed with a five-cylinder structure.Firstly，the piston position was monitored by a displacement sensor and the telescopic characteristics of the multicylinder were assumed，so that the piecewise method of buo-yancy and the maximum possible error range were obtained.Then，based on the design and experiments of the prototype，the fact that the multicylinder tends to retain the characteristics after running was verified.Meanwhile，the piecewise method of buoyancy calculation was optimized to improve precision ability largely.Experimental results show that the prototype can meet the requirements of the continuous buoyancy regulating of 0~80L，the minimum regulating step of 1L，and the comprehensive precision of no less than 0.5%FS.