The invention discloses a deep sea plug-in lithium battery power supply unit, and a system, and belongs to the technical field of seabed energy. The deep sea plug-in lithium battery power supply unitcomprises a plug-in self-compensation cell box, compensation membrane, hydraulic oil, electrical hydraulic watertight connectors, a lithium battery, a battery management system, and a sensor; the lithium battery, the battery management system, and the sensor are arranged in the plug-in self-compensation cell box; the internal part of the plug-in self-compensation cell box is filled with hydraulicoil; the compensation membrane is fixedly arranged on the top end of the plug-in self-compensation cell box using press strips and bolts; the plug-in self-compensation cell box is provided with a swallowtail clamping groove, and a swallowtail clamping part; the lower part of the plug-in self-compensation cell box is provided with two electrical hydraulic watertight connectors. The deep sea plug-inlithium battery power supply unit is capable of solving problems in the prior art that deep sea lithium battery system weight is high and size is large, and maintenance and replacing are inconvenientafter damaging; the modularization degree of the deep sea lithium battery power system is increased, and maintenance and replacing are convenient.