derivation of the conic coefficient error of practical aspheric optic surface is quite significant to aspheric machining accuracy, optical system imaging quality analysis and decomposition analysis of optical lenses. the primary mirror of r-c telescope system was tested by taylor hobson talysurf. the practical surface was fitted using zernike polynomials based on the date measured from talysurf. though taking the zernike coefficients into the optical system, the effect of the aberration which was brought by optical machining to the optical system imaging quality was obtained. the analysis shows that the spherical aberration was brought into the optical system because of the figure error of the primary mirror. and the value of the spherical aberration was same to the practical alignment result. then the conicoid aspherical degree of the primary mirror was tested by the talysurf. the machining deviation of the conic coefficient was gotten though comparing the conicoid aspherical degree of the practical primary mirror with that of the perfect primary mirror. the practical conic coefficient was calculated by the deviation. taking the practical conic coefficient into the r-c telescope system, the degradation of the optical system imaging quality was known. also the spherical aberration was brought into the optical system. experimental results show that the value of the spherical aberration analyzed by the two methods is same and consist with the practical alignment result. that is to say that the conic coefficient changed due to machining error of the conicoid aspherical degree. because of the change the spherical aberration was attached to primary mirror. and which caused the optical system imaging quality declined. finally, corrector was designed to balance the spherical aberration of the primary mirror. ensure that the optical system imaging quality meet the requirement.