In the application of low loss beam waveguides and high performance reflector antennas, high power microwave should be converted to quasi-Gaussian HE11 mode, to decrease side lobe level and increase feed coefficient. Based on the fact that TM01 mode converts to TE11 and TM11 mode at the same time in bent overmode circular waveguide, a TM01 to HE11 mode converter was proposed in this paper, which can convert TM01 mode to HE11 mode directly using a bent circular waveguide, the converter has higher power compacity than corrugated TE11-HE11 mode converter and more compact than TE11-TM01 mode converter added with smooth wall TE11-HE11 mode converter. The geometric parameters of the convertor were analyzed and optimized by using coupled mode theory and Taguchi method, to output the target modes content, 80% TE11 and 20% TM11 modes, which are primarily composed of HE11 mode. The optimized converter was analyzed using full electro-magnetic simulation method, the scalar Gauss ratio of the output beam is higher than 99% from 9.05 GHz to 9.8 GHz, the power capacity is as high as 4.5 GW.