The invention relates to a device and method for simulating a full-ocean-depth pressure battery performance. The device is composed of an iron-casting pressure container, a pressurizing mechanism, a to-be-tested soft lithium iron phosphate battery arranged inside the iron-casting pressure container, an insulating medium and a sensing mechanism. The iron-casting pressure container includes a pressure tank with a pressure gauge and pressure relief valve; and a circuit plugging port, the pressure gauge, and a pressurized pipe plugging port are arranged on the top of the iron-casting pressure container with the 140-MPa pressure-resistant performance. Therefore, a full-ocean-depth environment with the 100-MPa pressure and low temperature can be simulated; the lithium iron phosphate battery withpoor stability can be pressurized to be at the voltage of 100MPa; and the changing situation of electrical parameters of the bathyscaphe battery with depth changes is tested.