The air filter, pressure reducing valve and oil mist are assembled together, which is called pneumatic triplet. It is used to purify, filter and depressurize the air source entering the pneumatic instrument to the rated air source pressure supplied by the instrument. The pressure reducing valve can stabilize the pressure of the air source, keep the air source in a constant state, and reduce the damage to the valve or actuator and other hardware due to the sudden change of air pressure of the air source. The filter is used to clean the air source. It can filter the water in the compressed air and prevent the water from entering the device with the gas. The oil mist can lubricate the moving parts of the machine body and the parts that are inconvenient to add lubricating oil, which greatly prolongs the service life of the machine body.