A kind of fundamental mixer integrated circuit design under thick GaAs substrate is introduced by coplanar waveguide transmission line. The I-U and C-U curves are measured by a semiconductor, and the Schottky diode model is successfully extracted. Schottky diode is modeled and the Lange coupler, Intermediate Frequency(IF) circuit and match network are designed in zero-IF 140 GHz fundamental mixer on-chip circuit. Simulation results show that under a fixed intermediate frequency of 1 GHz, the best conversion loss is 7 dB, and 3 dB bandwidth is greater than 40 GHz.