External rear view mirror is attached at the side of the vehicle which is to permit clear vision for the driver to the rear of the vehicle. When the vehicle is running, the flow field around external rear view mirror is highly three-dimensional, unsteady, separated and turbulent which is known to be a significant source of aerodynamic noise and a contributor to the total drag force on the vehicle. While among all the researches on the flow field around external rear view mirror, different installation environment were employed. The external rear view mirror is mounted on a production car in most researches which presents the real condition and it can also be mounted on the ground of a wind tunnel, a specially designed table, or a generic vehicle model based on the SAE model. While, the relationship between the flow field around external rear view mirror and the installation environment is not very clear. A new open-source realistic car model which is called the DrivAer model is used as the baseline. The external rear view mirror of the DrivAer model is also mounted on SAE model, a designed table and the ground of the wind tunnel as contrastive cases. The hybrid RANS-LES method (DES) is used to investigate both the time averaged and unsteady flow field around the external rear view mirror. When the external rear view mirror is mounted on the realistic car model, the approach flow is accelerated and deflected. Vortex shedding exists in the near wake of the mirror and an anticlockwise swirling structure is formed in the far wake. Both the time averaged and unsteady flow field are different in other three cases. Further the influence of the installation environment on the flow field around external rear view mirror is analyzed.