The invention provides a vehicle-mounted temperature reduction device for a car, and relates to the technical field of refrigeration equipment. The vehicle-mounted temperature reduction device comprises a main box body and a water box, wherein the main box body is provided with an air temperature reduction loop and a liquid cooling loop; the two groups of loops are sealed pipelines; the air temperature reduction loop and the liquid cooling loop are in mutually laminated arrangement in the space of the main box body; a phase change material is arranged on the periphery of a pipeline in the main box body; the water box is arranged at one side of the main box body; a machine case is arranged at the lower part of the water box; one end of the air temperature reduction pipeline is connected with an air outlet of the space in the car; the other end of the air temperature reduction pipeline is connected with an air compressor; an air inlet of the air compressor is arranged in a space in the car; a water inlet of a water pump is connected with the water box; a water outlet of the water pump is connected with one end of the liquid cooling pipeline; and the other end of the liquid cooling pipeline is connected with the water box. The vehicle-mounted temperature reduction device is mainly used for reducing the temperature inside the car.