With the help of an electromagnetic field simulator(i.e., HFSS) and a lumped gap source method similar to the quasi optical antennas, the embedding impedance of a 660 GHz waveguide SIS (Superconductor Insulator Superconductor) mixer was thoroughly investigated from 600 to 720GHz.The effects of the junction's feed point displacement(including horizontal and vertical offsets), chip thickness and backshort length were analyzed and calculated.The results indicate that the simulated embedding impedance of the mixer is around 35Ω over the working frequency range, which can match the SIS junction in a large bandwidth. The feed point location of the SIS chip has little effect on the embedding impedance, while the chip thickness obviously does. These results will benefit the development of waveguide SIS mixers.