The earth-observing remote sensing micro/nano satellite with low-cost and high-performance is the focus of the current research and development. In order to adapt to the micro/nano satellite′s platform, the camera is often required to be compact and light weight. Limited of its volume and weight, designing a long focal length, wide-field telescope as compact as possible is the key of successfully manufacturing the high-resolution remote sensing optical camera. The possibility of solving this problem by using on-axis four-mirror system is studied. First of all, the basic composition and the working principle of on-axis four-mirror system without secondary obscuration was introduced. Then, the conditions of geometric constraint and the formulas of primary aberrations were presented based on Gauss and optical aberration theory. The design idea and method of the compact on-axis four-mirror system with long focal length were presented. Finally, an optimized optical system was given, with full field angle is 1°×1°, effective focal length (EFL) is 2 100 mm, F number is 7, respectively. The total length of this system is 228 mm, only about 1/9 of EFL. This optical system is simple and compact and its imaging quality is near diffraction limit.