In this study, an angle-related fitting function wherein the fitting coefficient is calculated using the least square fitting polynomial method is established for an active self-built dual-focus terahertz imaging system. In addition, the geometric optical tracking calculation process for the scanning track is successfully simplified into a binary function. The system adopts the same channel image scanning technique using a rotating mirror. Because the fitting independent variable has a significant influence on the terahertz image, two different fitting independent variables are used for calculation, and the advantages and disadvantages of the two are compared through numerical and imaging results analyses. The experimental results show that when the coordinate component of the normal vector of the rotating mirror is used as the fitting independent variable, the calculation time of the scanning trajectory is reduced to 3 s/frame, and the terahertz image is very accurate. Moreover, the system realizes fast and accurate imaging under a spatial resolution of 3 cm and view field of 50 cm × 100 cm when placed at a distance of approximately 8 m. Thus, this study provides a valuable reference for quick scanning in terahertz imaging systems and is of practical significance for real-time terahertz security inspection and non-destructive testing.