Vehicle handling stability is an important part of the vehicle performance. In the vehicle suspension and subframe system, subframe is often studied as a rigid connection, however, the front subframe undergoes elastic deformation in actual driving process. Therefore, this paper flexibly treats the front subframe of a certain vehicle and establishes a rigid-flexible coupled vehicle model to study the handling stability of the whole vehicle. The lateral dynamics of the whole vehicle is analyzed, the three-degree-of-freedom vehicle kinematics equations are derived, and the vehicle kinematics simulation is conducted for comparative analysis to study the influence of the front subframe flexibility on the KC characteristics of the suspension system and the transient handling stability of the whole vehicle. The NSGA-II algorithm is applied to optimize the bushing stiffness at the front subframe joint, which improves the transient handling stability of the whole vehicle. The optimization results show that at 0.5 Hz, the yaw rate gain relative to the steering wheel angle is reduced by 8%, the body roll angle relative to the lateral acceleration gain is reduced by 1.1%, and the lateral acceleration delay time relative to the steering wheel angle is reduced by 10.5%.