The primary method for calibrating a co-axial three-mirror system is the computer-aided alignment method,and it is complicated for having to calculate wavefront error. Stochastic parallel gradient descent (SPGD) control algorithm is used to caliberate the second mirror of three-mirror system,with no need to calculate wavefront error. Through adjusting the six degrees of freedom of the second mirror directly,finding system extremal value and then calibrating system error,the calibrating process is predigested. The major factors that will affect computing speed of SPGD algorithm are analysed and optimized,and the simulation results show that SPGD algorithm can distinctly improve peak-valley value and root-mean-square of the system.