A distributed sliding mode control algorithm has been proposed for high-speed trains (HST) based on extended sliding mode disturbance observer (ESMDO) in view of a solution of asynchronous speeds in each carriage caused by unknown external disturbances during high-speed train operation. Firstly, considering the stress condition of high-speed train operation, the multi-particle model of high-speed trains is established. Secondly, a sliding mode controller for the leader carriage and a distributed sliding mode controller for the follower carriage are designed, and an extended sliding mode disturbance observer (ESMDO) is constructed to estimate unknown external disturbances, thus providing feedback to the distributed sliding mode controller to eliminate the influence of unknown external disturbances on the speed synchronization of each carriage of the high-speed train. Meanwhile, the stability of the algorithm is proved by using the Lyapunov theory. Finally, it is verified that the algorithm can accurately observe the unknown external disturbances in real time and provide feedback compensation by Matlab simulation. The follower carriage can quickly track the speed of the leader carriage, with a speed tracking accuracy as high as 99.9%.