The invention relates to a mixed gas feeder of a target knife and a method for controlling refrigeration capacity output. The feeder comprises two steel cylinders, two gas flow controllers, a mixing chamber, a high-voltage pressure controller, a high-voltage magnetic valve, a low-voltage pressure controller, a low-voltage magnetic valve, a controller, a pre-cooling system and a target knife, wherein different gases with the Joule-Thomson effect are filled in the two steel cylinders; the gases flow out of the steel cylinders, enters the mixing chamber to be fully mixed after being subjected to flow regulation by the flow controller, flows through the pressure controllers and the magnetic valves in sequence, enters the pre-cooling system to be pre-cooled, and flows to a target zone for throttling cooling. According to the invention, the flow of the two steel cylinders can be regulated to be different through the controller so as to realize the situation that two gases are mixed in an arbitrary mixing rate; the throttling cooling property of N2 and Ar is fully utilized, so that the cooling capacity of an input knife head is accurately controlled, different cooling capacity requirements for cryoablation of tumors at different parts are met, the cryoablation effect is guaranteed and effective utilization rate of the cooling capacity of the pre-cooling system is improved at the same time, the gas source is saved, and burdens of a patient are alleviated.