The steady lateral acceleration based on road adhesion is usually denoted as the steer threat number, neglecting the limitation of vehicle characteristics on vehicle steering capability as well as the nonlinear and transient characteristics of high-speed steering maneuvers. To solve the issue, it is proposed to simulate the steering collision avoidance process by the steering wheel step test to obtain the maximum steady achievable lateral displacement as the steering threat number. Firstly, the critical stable lateral acceleration of the vehicle was determined by analyzing the tire lateral deflection and the vehicle characteristics. Secondly, the MAP of “lateral acceleration-longitudinal velocity-front wheel angle” was established according to the feedforward control algorithm. Thirdly, a steering feedforward control method considering lateral load transfer was proposed to improve the accuracy of the feed-forward front wheel angle in the steering wheel step input test. Finally, the stepping steering conditions under critical stability of the vehicle were established according to the MAP, and the nonlinear two-degree-of-freedom vehicle model was used for stepping steering simulation to obtain the maximum steady achievable lateral displacement plots. The proposed threat number and the steer threat number (STN) were compared and validated by the Monte Carlo method. The simulation results show that the proposed threat number can be more accurate in determining whether the vehicle can avoid collision by steering under extreme conditions compared to STN.