The invention provides a drainage type buoyancy regulating device. The buoyancy regulating device comprises a motor bracket, a motor, a band-type brake machine, a coupling, a ball screw, a ball screw pair, end covers, a sliding screw, ripple sacs, bearings, press covers, fastening nuts, small sealing rings, a connector, large sealing rings and sealing pressure strips. According to the drainage type buoyancy regulating device provided by the invention, seawater in the ripper sacs can be easy to drain, and the drainage volume change of an underwater glider is realized, so that the buoyance is regulated. The drainage type buoyancy regulating device has the advantages of being simple in structure, easy to operate, small in occupied space, good in reliability and the like.