The current research of real-time observation for vehicle roll steer angle and compliance steer angle(both of them comprehensively referred as the additional steer angle in this paper) mainly employs the linear vehicle dynamic model, in which only the lateral acceleration of vehicle body is considered. The observation accuracy resorting to this method cannot meet the requirements of vehicle real-time stability control, especially under extreme driving conditions. The paper explores the solution resorting to experimental method. Firstly, a multi-body dynamic model of a passenger car is built based on the ADAMS/Car software, whose dynamic accuracy is verified by the same vehicle’s roadway test data of steady static circular test. Based on this simulation platform, several influencing factors of additional steer angle under different driving conditions are quantitatively analyzed. Then ε-SVR algorithm is employed to build the additional steer angle prediction model, whose input vectors mainly include the sensor information of standard electronic stability control system(ESC). The method of typical slalom tests and FMVSS 126 tests are adopted to make simulation, train model and test model’s generalization performance. The test result shows that the influence of lateral acceleration on additional steer angle is maximal(the magnitude up to 1°), followed by the longitudinal acceleration-deceleration and the road wave amplitude(the magnitude up to 0.3°). Moreover, both the prediction accuracy and the calculation real-time of the model can meet the control requirements of ESC. This research expands the accurate observation methods of the additional steer angle under extreme driving conditions. The current research of real-time observation for vehicle roll steer angle and compliance steer angle (both of them comprehensively referred to as the additional steer angle in this paper) mainly employs the linear vehicle dynamic model, in which only the lateral acceleration of vehicle body is Viewed. The observation accuracy resorting to this method can not meet the requirements of vehicle real-time stability control, especially under extreme driving conditions. built based on the ADAMS / Car software, whose dynamic accuracy is verified by the same vehicle’s roadway test data of steady static circular test. Based on this simulation platform, several influencing factors of additional steer angle under different driving conditions are quantitatively analyzed. Then ε -SVR algorithm is employed to build the additional steer angle prediction model, whose input vectors m ainly include the sensor information of standard electronic stability control system (ESC). The method of typical slalom tests and FMVSS 126 tests are adopted to make simulation, train model and test model’s generalization performance. The test result shows that the influence of lateral acceleration on additional steer angle is maximal (the magnitude up to 1 °), followed by the longitudinal acceleration-deceleration and the road wave amplitude (the magnitude up to 0.3 °). Moreover, both the prediction accuracy and the calculation real-time of the model can meet the control requirements of ESC. This research expands the accurate observation methods of the additional steer angle under extreme driving conditions.