Abstract: When electromagnetic wave flies through the interplanetary space region, the solar plasma will bring large reflection loss to the TT&C communication signal and reduce the power of the TT&C communication signal. The influence of time delay error of electromagnetic wave on ranging can reach tens of meters in one way. In this paper, the formation principle of the solar plasma region was analyzed, the relationship between the plasma delay error and the angle of Sun-Earth-Probe（ SEP ）and the distance r between the detector and the sun center was simulated, and the influence of the solar plasma time delay of Tianwen-1 on ranging was discussed. The results show that the plasma delay decreases with the increase of the SEP and r , and the effect of solar plasma delay on ranging is less than 2 m due to the SEP more than 20° in the Earth-Mars transfer orbit and the initial orbit of the circum-Mars, while the two-way effect of solar plasma time delay on ranging increases gradually from about 2 ～ 100 m when the SEP is less than 20° and close to 0°. The delay effect of solar plasma must be considered in precise orbit determination. After plasma delay error correction of ranging data, the residual error can be eliminated by dynamic orbit improvement, and the final residual error of orbit determination can reach within 4 m.