Abstract: Superconducting hot electron bolometer mixer which is the highest sensitive coherent detector above 1 THz. In order to further improve the sensitivity and effectively reduce the loss of RF signal in the transmission path in ultrawide band, it is important to design a RF matching circuit with high coupling efficiency. This paper firstly presents a theoretical model for the feed point matching impedance, and employs a 3D electromagnetic simulation software (HFSS) to design an ultrabroadband and lowloss THz signal coupling circuit based on lumpedport source method similar to quasi optical antenna. Embedding impedance of the superconducting HEB mixer in the 0.9~1.3 THz has been studied in detail. The effect of low pass filter composed of high and low impedance lines, highorder modes in the probe structure from waveguide to microstrip, and chip thickness on the embedding impedance are investigated. In particular, we study the influence of slice tolerance of substrate thickness and feedpoint size on the embedding impedance. The mixer embedding impedance is simulated to be around 35Ω-j10Ω in the whole operation band. The mixer relative bandwidth can reach 36%. The simulation results are in good agreement with the calculated results according to the theoretical model. The calculation results provide helpful instruction for the future development of ultrawide band and highly sensitive superconducting HEB mixer.