R-C optical system was a commonly used optical system in modern optical engineering, a case study of alignment technique of R-C optical system was stuided. The alignment theory of R-C optical system was analyzed. The main mirror was the adjustment datum, the five degree of freedom of secondary mirror was adjusted to improve the wavefront error of the whole optical system. The relationship between secondary mirror misalignment and Zernike coefficient of the whole system was obtained by computer-aided alignment and was used to guide alignment of secondary mirror. By use of this method, the alignment purpose was very clearly, that was adjusting the position of secondary mirror to reduce Zernike coefficient. The alignment period was then reduced. After alignment, wavefront error of the whole optical system was better than 1/10λ(RMS). It satisfies the design requirement. The images obtained from laboratory and outfield indicate the alignment is valid.