The fundamental theory for coherent beam combining using SPGD algorithm is introduced. The evaluation function, perturbation mode and the distribution character of perturbation voltages are optimized by using numerical simulation. Coherent beam combining of three W-level polarize-maintained fiber amplifiers is accomplished by implementing SPGD algorithm on a digital signal processor. The experimental results demonstrate that the optical phase for each beam channel can be effectively controlled by the SPGD algorithm, the energy encircled in the target pinhole is increased by a factor of 2.62, and the combining efficiency is as high as 87% compared with its ideal counterpart when the system is in the close-loop.