The utility model discloses a pneumatic device of a main control valve of a liquid transfer box of a diesel locomotive. A main body is installed on the upper portion of a support, an upper cover is fixedly connected with the upper portion of the main body and a box body of the liquid transfer box, the upper cover is provided with a central hole, and a telescopic rod close to a switch is installed in the central hole. A first-gear piston and a piston sleeve which are matched with each other are installed on the upper portion in the main body, and a second-gear piston connected with the first-gear piston is installed on the lower portion in the main body. The low end of the second-gear piston is connected with a push rod of the main control valve. The pneumatic device has the advantages of being reasonable in structure and simple and practical in use. The upper cover installation mode replaces the existing support installation mode, and users only need to loosen or fasten a bolt of the upper cover in assembly and disassembly, so that installation and overhaul are facilitated. The users do not need to wait for time when oil temperature of the liquid transmission box drops below 40 DEG C when processing faults, so that the faults can be processed timely, the average 4 hours of overhaul of the pneumatic device and fault temporary maintenance are reduced to 2 hours, the time of the overhaul of the pneumatic device and fault temporary maintenance for the main control valve is greatly reduced, and labour intensity of maintaining workers is reduced.