Abstract: Radiator profile structure is complex and difficult to produce, how to correct design aluminum extrusion die is critical. In this paper, a radiator extrusion die is designed by experience. After tryout, the level degree is bad and profiles are not correctly assembled. In order to find the specific reasons, a numerical simulation is carried out to the initial design. A finite element analysis model is established, the extrusion processes in steady state and transient flow state are simulated. Profile exit displacement, metal flow velocity, temperature distribution, metal flow stress, transient pressure change and die temperature, die stress are analyzed, the affection of process parameters on the extrusion process is studied too. Based on analysis result and tryout, we propose to add flow-promotion angle in up die, to add baffles on the low die and to modify bearing length. The modified die effectively solves the initial die design problem which velocity distribution in exit section is not uniform. Simulation results are good agreement with practice, finally produced profiles are qualified and can be correctly assembled. Instead of the tryout, we can guide the die design by FEM (finite element method).