Eccentric error of planetary gear was transformed into equivalent mesh error on meshing line by geometry method. Then coupling error of every equivalent mesh error was calculated and deduced when angular frequency was 0, same and different. The best compensational effect was obtained with the first two cases. The value range of the difference among initial phase errors were concluded when the coupling error could be effectively compensated and the relationship of initial phase with the best compensation. Furthermore, assembling method by allocating initial phase error was proposed, which could improve transmission accuracy of planetary gear. The transmission errors of planetary gear were calculated with allocating assembly, random assembly and machining precision improvement, respectively. The comparison and analysis proved that the proposed method was effective and economical through. 