In order to estimate the parameters of the vehicle under cornering and steering conditions, the paper proposes two EKF-based observer models. The models consider the influence of the braking/driving force of each individual tire on the whole vehicle. For the purpose of improving the accuracy of the lateral force estimation, the lateral relaxation length is introduced and then observe the influence of relaxation length' change on observer through simulation. Finally, the estimated effect of the observers are verified by double lane-change test under the Co-simulation experiment of Carsim/Simulink. The simulation results show that the proposed method can effectively estimate the vehicle state parameters.