The invention provides a microwave tumor ablation instrument which comprises a microwave generation device, a power supply driving the microwave generation device to work, an ablation needle guiding microwave energy to an ablation structure, and a temperature measuring device used for detecting the temperature near the ablation structure. The instrument further comprises an anode current measuring device used for measuring the anode current value of the microwave generation device, and a fuzzy PID control device used for generating control signals according to the anode current value of the microwave generation device and controlling the phase angle of a phase control rectification device in the power supply so that the anode current of the microwave generation device can be equal to or close to a set value. The instrument effectively solves the technical problems that consistency of average power output by an existing microwave tumor ablation instrument is poor and/or instantaneous power consistency under the same average power is poor.