The invention provides a device and a method for preparing bulk nano-crystal magnesium alloy material based on nano-crystal hydrogenated magnesium alloy powder, relating to the preparation field of bulk nano-crystal magnesium alloy material, which solves the problem of lack of devices and methods for preparing the bulk nano-crystal magnesium alloy material. An outer casing formed by a bottom and a lateral wall is fixed on a base, the outer casing, a clapboard and a pressure head form a sealing space, an inner base is fixed on the bottom of the outer casing inside the sealing space, a thermal baffle is fixed on the inner base, a cavity die is fixed on the thermal baffle, a heating collar is fixed on the surroundings of the lateral wall of the cavity die, a thermoelectric couple is inserted into the cavity die, the clapboard is fixedly connected with the lateral wall of the outer casing through a sealing ring, the pressure ring is embedded into a through hole of the clapboard and pressed right above a punch, the sealing ring is fixed in a gap between the clapboard and the pressure head, and a vacuum pump port is fixed inside the through hole of the outer casing lateral wall. The method of the invention carries out vacuum dehydrogenation treatment to the nano-crystal hydrogenated magnesium alloy powder under certain temperature, applies pressure immediately and cools to obtain the bulk nano-crystal magnesium alloy material.