The invention provides a pressure reducing automatic control device of an air-cooling heat sink. The pressure reducing automatic control device comprises the air-cooling heat sink, wherein a pressurereducing device is arranged in the air-cooling heat sink, an automatic control device is arranged on the pressure reducing device, the automatic control device comprises a pressure sensor, and a sensing switch for controlling the running of the pressure reducing device is arranged on the pressure sensor. When pressure in the air-cooling heat sink exceeds a rated value, the pressure reducing deviceruns under the control of the sensing switch. When the pressure in the air-cooling heat sink returns to the rated value, the pressure reducing device stops running under the control of the sensing switch. The pressure reducing device comprises an evaporator for converting air pressure into liquid and further comprises an air return pipe communicating with the evaporator, wherein the air return pipe is connected with a compressor, and a draining pipe led outside the air-cooling heat sink is arranged on the compressor; and the pressure sensor is arranged on the evaporator, and the sensing switch is connected with the evaporator. According to the product, one automatic control device is additionally arranged in the air-cooling heat sink, and the pressure in the air-cooling heat sink can be stabilized at 10kg/cm<2> more or less, so that an ideal effect is realized, the service life of the air-cooling heat sink is prolonged, and the practicability is high.