The utility model aims to provide a frequency conversion device for a screw-type air compressor, and discloses an energy-saving control device for the air compressor. The energy-saving control device is characterized by mainly comprising the air compressor, a frequency converter, a power frequency contactor, a judging protection device, an air storage tank and a pressure transmitter, wherein the frequency converter and the power frequency contactor are connected with one end of the judging protection device; the other end of the judging protection device is directly connected with the air compressor; the outlet of the air compressor is directly connected with the air storage tank; and the pressure transmitter in circuit connection with the frequency converter is arranged in the air storage tank, and used for transmitting detection signals to the frequency converter. The frequency conversion device has the benefits that through the additional arrangement of the energy-saving control device, the energy consumption of the air compressor is reduced, the energy is saved, and the environment is protected.