A pillbox-type radio-frequency window for lower-hybrid current drive power transmission of 3.7 GHz for 200 kW/2 s is designed. The relative permittivity and the loss tangent of several domestic materials—alumina, boron nitride, and sapphire—are exactly compared by the rectangular cavity perturbation method, and finally, the sapphire is chosen as the window medium. The reflection coefficient of the optimized window can reach 55−dB at 3.7 GHz simulated by high-frequency simulation software, and the peak temperature rise can be limited at 20°C with maximum thermal stress of 1.7 MPa by thermal and mechanical analysis. In the high-power test, 221 kW/3 s energy passes the welded window.