A fundamental way to improve the ability of lidar to detect the small target far away is to increase the light intensity on it, and the coherent combination of laser beams is an effective way to obtain a large intensity on the target. A parameter named combining effect factor is introduced to evaluate the combining effect on small target. On this basis, the factors influencing combining effect are discussed. The combining effect factor becomes oscillatory as the spacing between the waists of laser beams increases, and its amplitude decreases gradually. The distance at which an ideal combining effect can be achieved increases with the increase of the spacing between the waists of laser beams, which is a reference for the determination of the spacing between the waists of laser beams in the design of coherent combining system. The dependence of combining effect on the phase control accuracy is also studied. The combining effect can reach 80 percent of its ideal value when the phase control accuracy reaches /4, and the combining effect decreases to 50 percent of its ideal value when the accuracy is /2. The influence of unparallel polarization direction between beams on combining effect is not very obvious.