The invention provides an optical system for stereo mapping with a low base-height ratio, which is used in a bias-field, adopts a coaxial tri-mirror system in which a primary mirror, a secondary mirror, a tertiary mirror and a reflecting plane mirror are completely shared, and is formed by combining two independent optical imaging paths. Two groups of incident light rays each of which has an included angle of 1 to 6 degrees sequentially reach the primary mirror, the secondary mirror and the tertiary mirror; and finally, the incident light rays are respectively reflected to two independent receiving image planes for imaging by the reflecting plane mirror. By using the bifocal surface as well as the primary mirror, the secondary mirror, the tertiary mirror and the reflecting plane mirror which are simultaneously shared, the invention is capable of simultaneously imaging in front and rear fields of view, therefore, the stereoscopic mapping with a low base-height ratio and high precision can be achieved by the reflection-type optical system capable of imaging on a single-camera and double-channel basis. The optical system has the advantages of high optical-mechanical structural integrity, small size, light weight, convenient real-time on-orbit detection and high stability of inner orientation elements. Therefore, the optical system is particularly applicable to single-track push-broom stereo imaging and mapping by a satellite camera in surveying and mapping with a low base-height ratio.