Abstract: The combination effect was evaluated by combining effect factor defined as the ratio of actual maximum light intensity to the ideal combined light intensity at the target. On this basis，the main factors affecting combination effect and the propagation property of combination effect factor were studied．Good combination effect could be obtained with a small duty ratio，while the main factor influencing the combination effect is the random phase errors，because the beam combination has actually become incoherently combined when the random phase errors are greater than a specific value．With the increase of the propagation distance，the influence of the duty ratio on combination effect becomes smaller, and one of the main factors influencing combination effect is random phase errors.