The axial static pressure coefficient is the key parameter to estimate the flow quality of automotive wind tunnel test section. The collector angle has an influence on the axial static pressure coefficient. However, the tendency of the axial static pressure coefficient isn’ t the same for different wind tunnels. According to the numerical results of different wind tunnels at different diffuser angles, it is found that the axial static pressure coefficient can’t be changed at the diffuser angle of 1.70°. When the diffuser angle is below 1.70°, the bigger collector angle becomes, the larger axial static pressure is. On the contrary, when the diffuser angle is above 1.70°, the law is opposite. The reason is that the constant dissipation angle exists at the shear layer. When the diffuser angle is equal to the dissipation angle, diffuser effect due to the bigger collector angle is the same as the effect due to the more flow entered from outside. If the diffuser angle is smaller than dissipation angle, the diffuser effect is more obvious. All those can be revealed the inconsistent mechanism for different wind tunnels. These findings can help to decide the key dimensions of test section and collector at the stage of wind tunnel construction.