The invention discloses a steam turbine dry-wet mixing frequency converting control air extracting system and an operation method thereof. The steam turbine air extracting system comprises a pressuretransmitter, a dry vacuum pump, a wet vacuum pump, an air water separator and a frequency converting control system. The frequency converting control system is adopted for adjusting operation of the steam turbine air extracting system, vacuum can be established rapidly during starting of a steam turbine, and air is extracted scientifically and reasonably during normal operation; and the problem that the air extracting capacity of existing steam turbine air extracting system is limited by water temperatures is solved. The rotation speed of the wet vacuum pump can be flexibly controlled, the compression ratio distribution is flexibly adjusted, and the air extracting compression ratio change range is enlarged.