The invention discloses a full depth buoyancy regulating seawater pump, which mainly comprises a flat valve module, a plunger piston shoe module, a plunger sleeve, a force transfer and reset mechanism and a pressure compensator, wherein the pressure compensator is used for balancing a lubricating cavity and environmental stress; the force transfer and reset mechanism drives the plunger piston shoe module to alternately cycle for a pressing travel and a sucking travel, so that the flat valve module alternately cyclically sucks water and presses water; the plunger piston shoe module has a fixed interval forced reset structure, so that a plunger is reset reliably; the force transfer and reset mechanism has a bidirectional stress structural design, so that the seawater pump meets the special working condition of regulating the buoyancy by the hydraulic pressure of seawater; and a stepped plunger is adopted, large and small plungers are connected through a spherical hinge, and the problems of strength design, sealing and frictional wear of the crucial friction pair, namely an ultrahigh pressure seawater pump are solved. The full depth buoyancy regulating seawater pump can regulate the buoyancy of a submersible in sea areas with all depth, and has the characteristics of simple structure, high applicability and working reliability.