In order to improve the accuracy of numerical simulation for injection molding process, a modified method for outlet condition was introduced. As the feedstock is regarded as incompressible fluid, the filling ratio should be a linear one with respect to time. But there remains a persistent trouble in previous researches that the linearity is not respected when the filling front approaches near the outlet boundary. The problem is caused by lack of adequate treatment on the outlet boundary. To remedy this defect, the present paper deals with the modeling and realization of suitable condition on outlet boundary for solution of the whole filling process. A simple straight channel mold was taken as an example to prove the proposed simulation method. The result shows that this modified method can suppress the distortion phenomenon and can be valid to realize the correct simulation for the filling of incompressible viscous flow at the ending stage. This long-term filling problem was finally solved.