Vehicle active collision avoidance includes collision avoidance by braking and by steering. However, both of these two methods have their limitations. Therefore, it is significant to establish the feasible region of active collision avoidance to choose the optimal way to avoid traffic accidents.This paper focuses on the steering control of an autonomous vehicle to track the planned trajectory and to perform an emergency collision avoidance maneuver. Meanwhile, the collision avoidance effect of steering control is compared with that of braking control. The path tracking controller is designed by hierarchical control structure. The upper controller includes model predictive control allocation and speed controller, and the lower is designed by weighted least-squares control allocation for torque allocation. Besides, seven order polynomial is used for path planning. In this paper, two kinds of steering collision avoidance measures are compared, including steering without yaw moment and with yaw moment. The result shows applying the additional yaw moment can assist the vehicle to steer when the steering input reaches the limit value. Finally, the feasible collision avoidance regions based on the braking/steering are built under the conditions that the front car is stationary, in constant speed or in constant deceleration.