The utility model relates to a horizontal type high-pressure jet cleaning gun for completely cleaning an oil storage tank. The cleaning gun comprises an air motor, a retarding mechanism, a transmission shaft, a swiveling mechanism, a swinging air cylinder, a nozzle connector and a nozzle, wherein the air motor is connected with the retarding mechanism, a gear in the retarding mechanism is fixedly connected with one end of the transmission shaft, the other end of the transmission shaft is connected with the swiveling mechanism, the swinging air cylinder is arranged on the swiveling mechanism, the transmission shaft of the swinging air cylinder is connected with a hollow mandrel, the other end of the hollow mandrel is connected with one end of the nozzle connector, and the other end of the nozzle connector is connected with the nozzle; a water pipe is externally sleeved on the transmission shaft, the side face of the water pipe is provided with a water inlet, the inner wall of the water pipe is connected with an adapter sleeve, the adapter sleeve is connected with the swiveling mechanism, the swiveling mechanism is provided with a water port, and the hollow mandrel is communicated with the water port of the swiveling mechanism. The horizontal type high-pressure jet cleaning gun realizes mechanical motion of the gun body by pneumatic control, and jets high-pressure fluid out of the gun body from different angles, thereby completely cleaning the oil storage tank.