The invention provides an optical system for a miniaturization high-precision star sensor. According to the invention, a quasi-gauss lens structure is adopted and formed by 6 lenses; all the lenses are spherical lenses; and the glass of the first lens is made of fused quartz materials so that the first lens can be immediately used for correcting image difference and can be used as protection glass of the optical system. According to the optical system is characterized by wide spectrum, large view side and large relative aperture and has quite small distortion quantity in a quite wide spectrum range and view field; the concentration degree of confusion disc energy of each view field is uniformly distributed; the imaging quality is good; the system can work at a temperature from -50 DEG C to +70 DEG C and has good image quality and defocusing amount; and posture measurement of high-precision star sensor working in severe space and temperature can be met.