In hybrid energy storage systems, state estimation is the basis for power allocation and control strategy adjustment among the storage elements. In order to reduce the impact of state estimation errors on the subsequent energy management, a hybrid energy storage system "li-ion battery-supercapacitor" is used for modeling and simulation and accurate state estimation. Firstly, after identifying the basic component parameters, a simulation model of the hybrid energy storage system is built in MATLAB/Simulink environment based on the physical structure of the circuit for fast and accurate simulation of the physical experiment. Then, considering the complexity and the accuracy of estimation, the battery pack and the supercapacitor pack are modeled in a holistic form, and the joint estimation of charge state and health state is carried out with the "EKF-two-point method". Finally, the experimental verification shows that the errors of the state estimation are within 5%, i.e., the simulation model established in this paper can accurately reflect the operating characteristics of the components and the accuracy of the joint state estimation algorithm is high.