The transient response characteristics of a vehicle under steering angle input are important evaluating indicators of vehicle handling stability. For a new developed vehicle, which was found that the transient response under steering angle input is too slow at high speed, a rigid-flexible coupling vehicle model is established in ADAMS/Car based on multi-body dynamics theory. Improvement measures are studied and put forward to improve the transient response characteristics of the vehicle. The sensitivities of transient response to various parameters are analyzed. The optimization method of adjusting the tire cornering stiffness and moving forward the mass center is adopted. The test data after improvement show that the response time of yaw velocity is shortened obviously. Meanwhile, the value of evaluation index in other tests remains basically unchanged. Following conclusions can be drawn from this paper. 1) Using rigid-flexible coupling method, combining with mechanics parameters which was obtained from actual measurements, a vehicle model with good accuracy can be established which can duly reflect the dynamics characteristics of a vehicle. 2) By adjusting the parameters in the rigid-flexible coupling vehicle model, the sensitive factors of transient response time under steering angle input can be identified quickly.