Based on the coupling wave theory and phase re-matching technique, a Q-band TE01-TE11 circular waveguide mode converter for transmitting high power microwave has been designed and analyzed by numerical calculation and simulation. Results of 4-period and 6-period mode converter are demonstrated in this paper. The numerical calculation show that maximum conversion efficiency is 98.82% and bandwidth is over 2.2GHz for 4-period structure. However, for the 6-period one, the maximum conversion efficiency is 99.89% and bandwidth is over 1.7GHz. It also presented the influence of the key parameters on the conversion efficiency. Simulated results from HFSS agreed well with the numerical calculation. Based on the theory and simulated results, a mode converter was fabricated and tested. It turned out that a pure TE11 mode was abtained.