A two-stage medium-pressure screw air compressor includes a primary screw compressor (21) and a secondary screw compressor (22) communicating with the gas-liquid separator (6) via the first gas-liquid inlet (61). A first oil outlet (63) is connected to a mid section of a gas-liquid separator (6). The oil is injected into the lubricating components of the compressor is the first oil outlet (63), a first filter (64). A secondary oil-water separator (9) is connected to the first oil outlet (63) of the gas-liquid separator (6), the second oil-water separator (6) has an upper separating tank (91) and a lower separating tank. The water and the impurities contained in the lubricating oil can be eliminated at a largest extent, so as to create essential condition thr ensuring the screw compressor to operate continuously, stably and safely and provide high operational reliability, safety and long service life.