Abstract: This paper studies the relation between the non-parallel flow and the gradually varying flow in gradually contraction plates by numerical simulations. The influence of the incline angle of the plates on the flow is analyzed. According to the data obtained by means of the EXP(C_ES) mode, a one per cent relative deviation is seen between the non-parallel flow at the incline angle of 5.455° and the parallel flow, while one sees a ten per cent deviation at 13.26°. The angle of 5.455° corresponding to a one per cent deviation is regarded as the critical value of the gradually varying flow. The reasonability and reliability of the numerical simulation of the flow between gradually contraction plates are discussed at the end of the paper.