The invention, which belongs to the star sensor technology, particularly relates to an optical system for a high-precision miniaturization star sensor in an airborne environment. The optical system is composed of an optical window, an oscillating mirror, a secondary mirror group, a secondary mirror frame, and a primary mirror. The secondary mirror group is fixed in the secondary mirror frame. The secondary mirror group and the primary mirror form a small-view-field convergence imaging system based on a cassegrain two-mirror system; and the optical axis of the optical window is perpendicular to the optical axis of the small-view-field convergence imaging system. The oscillating mirror is arranged on the optical axis of the small-view-field convergence imaging system in an inclined mode and faces the optical window in an inclined mode. According to the invention, the size of the airborne star sensor can be reduced to be the half or less of the size of the existing star sensor in the prior art; the imaging can be guaranteed to be in a diffraction limit state; and requirements of miniaturization and high precision can be met simultaneously. Besides, the optical system has advantages of simple structure, high reliability, low process manufacturing difficulty, and low cost and can adapt to the airborne working environment well; and the system that can be popularized and applied easily has the great practical value.