Abstract: This report studies the filling stage of Reaction Injection(RIM).A typical rectangular mold is used to simulate this filling stage.A model of this process is developed and solved by using the finite element method(FEM).The quadrilateral isoparametric element is used and the boundary grid line fits well with the flow front.ALE method which has the advantages of Lagrange method and Euler method is adopted to treat the flow front,and FEM mesh is created once the front moves to satisfy the boundary conditions of the front.Different filling parameters and mold thickness are chosen to calculate this stage.The conversion distribution curves are given and the results are analyzed.It shows that relatively thin mold thickness,rapid injection speed and high initial temperature can get uniform properties of the final product.The results of this paper are helpful in optimized control of the RIM mold structure and the filling parameters.