In order to obtain the optimal aerodynamic shape of auto rearview mirror and improve fuel economy, the CFD method is applied to numerical analysis on automobile external flow field. The interference drag properties of auto rearview mirror are described in detail and the influence of the mirror，s shape and its horizontal distance to the body on the aerodynamic drag is mainly analyzed. The result shows that the aerodynamic drag of auto rearview mirror for the whole is 4.08% and the influence of its shape and position is great, which at the best horizontal distance the aerodynamic drag coefficient can drop 2.76 %. The wind tunnel trial on car model verifies the accuracy of numerical computation method, and the results of the computationa analysis provide reference basis for the design and improvement of auto rearview mirror.