Considering the complexity of internal structure and optical path of the off-axis three-mirror system, it is difficult to directly analyze the effect of deflection error of each mirror inside the off-axis three-mirror system on the line-of-sight. Therefore, a method of simplifying the off-axis three-mirror system model was presented to a single-mirror system model for analysis. Then, based on the model of single reflection system, the formula of the deflection error of the off-axis three-reaction was derived. Finally, the model of off-axis three-mirror system was created by Zemax software, the change of the line-of-sight of the system was simulated by adjusting the position of the mirror and the coordinates of imaging can be obtained, then the deflection value of the system′s line-of-sight was solved by substituting the image coordinates into the calculation formula. From comparing the results, the maximum error of the deflection of the line-of-sight relative to the deflection of the line-of-sight in the simulation is not more than 4%, which verifies the correctness of the theoretical analysis based on the single reflection model.