The invention discloses a train, a train power supply system and a leakage protection device thereof, wherein, the power supply system comprises a power grid for supplying power to the train and a vehicle-mounted battery pack; the leakage protection device comprises a first circuit breaker connected with a positive pole of the power grid; A first positive electrode contactor connected to the firstcircuit breaker and the train receiving positive electrode; A first negative electrode contactor connected to the grid negative electrode and the train receiving negative electrode; A second positiveelectrode contactor connected to the battery positive electrode and the train receiving positive electrode; A second negative electrode contactor connected to the battery negative electrode and the train receiving negative electrode; A first leakage protection assembly for detecting whether leakage occurs at the high-voltage positive pole of the train; The controller is used for controlling the on-off of the circuit breaker and the contactors to realize the power supply of the electric network or the on-board battery pack to the train. When the electric network supplies power to the train andthe high-voltage positive pole of the train is leaked, the controller or the first leakage protection unit controls the first circuit breaker to be turned off, thereby improving the safety and reliability of the train operation.