The principle of Vlasov mode converter for whispering-gallery mode gyrotron was studied in detail by using a geometric optics model. The radiation field passing through the mode converter was analyzed by using the equivalent image source model and the vector diffraction theory. After considering some particular parameters of W-band whispering-gallery mode gyrotron, the field distribution of radiated beam passing through Vlasov mode converter was discussed by numerical calculation. The results indicate that the high-order whispering-gallery mode inside the gyrotron is converted into Gaussian beams in free space by the Vlasov mode converter, and the Gaussian distribution in the far zone is superior to that in the near zone.