As active correction of a large aperture SiC mirror with high stiffness is vulnerable to many calculation errors by direction least square method or free resonance method, this paper proposes a primary surface correction algorithm using bending mode to calculate and optimize the active force and to improve the correction capability. Firstly, a series of mathematical transformations were performed on the influence matrix of the primary mirror, and a set of orthogonal bending modes of primary mirror were obtained. Then, correction targets were fitted in bending modes to calculate the correction force. An 1.23 m SiC mirror and a support system were modeled by finite element analysis and the algorithm was verified by simulation experiments. Moreover, an active support system for the 1.23 m SiC mirror was set up to correct primary surface and a further optimization for the algorithm was conducted based on this system. The experiments show that the surface error is corrected from 0.23λ RMS to 0.048λ RMS by the proposed bending mode. Results of analysis and experiment demonstrate that the algorithm by bending mode efficiently reduces active force ranges and improves correction capability. It is significant for the active correction of large aperture SiC mirrors with high stiffness.