Based on geometry optics theory and vector diffraction theory, mode converter is studied to convert the gyrotron and other high-power microwave oscillator output mode into quasi-optical Gaussian beam, using Vlasov launcher and two quasi-optical reflector achieved quasi-Gaussian mode TEM00 transverse output.The working mechanism of Vlasov launcher in analyzed, the radiation field is calculated by using vector diffraction theory, and the surface radiation field is also calculated by using the surface current distribution method. A built-in quasi-optical mode converter is designed to convert the 94GHz, TE62 model millimeter wave into quasi-optical Gaussian beam through programming.