The design and experiment of 94 GHz gyrotron with an inside quasi-optical system for the non-lethal biological effects of millimeter wave radiation are presented in this paper. The reason we chose TE_+6,2 mode as the experimental working mode is to reduce the wall heating problem at high power and high frequency. For a high-order mode, there are more neighboring modes, so mode competition affects the stability and effective operability of the experiment. The gradually tapered cavity has been designed to suppress mode competition in single cavities. In addition, the power conversion efficiency of the quasi-optical mode converter with low diffraction is 98.54%. The experimental results confirm that the output power of 50.9 kW is obtained with efficiency of 34.3%. For the non-lethal biological effects, we can draw the conclusion that the overall scheme can achieved the expected results.