The filling process of plastic precise gear is simulated by CAE software and the factors that influence injection molding accuracy in single cavity mold are discussed in this paper. With the assistant of visualization technology, the mechanism of filling unbalance which influences efficient injection molding was especially studied. The model of filling unbalance was supposed that was verified experimentally by visualization technique. It is clarified that the mechanism of filling unbalance results from dissymmetrical temperature distribution on the cross-section of runner, which is strongly influenced by the shear heat and the cooling process in the runner. At the same time, a new method was proposed for improving the filling imbalance by applying appropriate injection molding process without any changes in runner shape of multi-cavity mold.