In this paper, a quasi-optical resonator with new coupling method and parasitic-mode-suppressing in 110~170GHz is proposed. The coupling of THz quasi-optical resonator in this paper is realized by PCB substrate processing and pressurized flange. At the same time, the parasitic mode is effectively suppressed by slitting the plane mirror to reduce the influence on the main mode during the test. And fused quartz sample was tested to verify the feasibility of the system for complex dielectric testing of materials in 110~170GHz.