LLC resonant converter, which is widely used in the charging of new energy vehicles, can achieve zero-voltage switching (ZVS) of the primary side MOSFET and zero-current switching (ZCS) of the secondary side rectifying diode. They also have the advantages of high efficiency and high-power density. The output voltage is still affected by input voltage fluctuation, reference voltage change, load mutation and other factors. The charging mode of automobile batteries with different materials is also different. In order to stabilize the output voltage and realize automatic switching of constant voltage and current, this paper proposes a double closed-loop linear active disturbance rejection control (LADRC) scheme. Firstly, the transfer functions of the system are obtained by extended description function (EDF) method and simulation method respectively. The transfer functions of the two methods are compared and analysed. Finally, the LADRC controller and PI controller are simulated and compared, and the dynamic performance of the control system is evaluated by ITAE index. The experimental results show that the output voltage of LADRC control is more stable.