Research on remote sensing imaging is advancing toward achieving higher ground resolution, wider fields of view, reduced weight, and improved imaging quality. Off-axis three-mirror optical systems have increasingly become a standard configuration for remote sensing cameras. However, the installation and alignment of such systems remain challenging, consequently hindering rapid prototyping and engineering implementation. Thus, this study proposes a method based on the angle between the optical axis and the installation reference surface of the aspheric component. First, non-contact measurement is employed to determine the angle between the aspheric optical axis and the installation reference surface. Subsequently, a theodolite is used to mark the zero-field-of-view reference, thereby minimizing optical axis transmission errors. Next, the secondary and tertiary mirrors are installed with high-precision coaxial alignment. Finally, the system is fine-tuned by adjusting the primary mirror. Using a low-light remote sensing camera as a case study, this method is applied to install and align an off-axis three-mirror optical system. Following installation and alignment, ground-based long-distance imaging tests demonstrates high imaging quality. In practical applications, this method enables precise pitch and yaw positioning of aspheric components, facilitates one-time high-precision coaxial alignment of the secondary and tertiary mirrors, accelerates system installation and alignment, reduces the degrees of freedom required for precision adjustments, and lowers overall installation complexity.