The invention discloses a trolley bus power system and a power-grid-connected operation method and a power-grid-free operation method thereof. The trolley bus power system comprises a motor controller connected with a bus of a whole vehicle and used for controlling a motor, wherein the bus of the whole vehicle is used for being connected with an external power grid. The trolley bus power system further comprises a vehicle-mounted battery, a battery management system of the vehicle-mounted battery, a super capacitor and a capacitor management system of the super capacitor. The motor controller, the battery management system and the capacitor management system are all connected with the controller in communication mode. On the basis of the existing hybrid power supply of an external power grid and the vehicle-mounted battery, the super capacitor is added, a characteristic of fast-charging fast-discharging of the super capacitor is used, three types of power supplies are controlled in coordinate mode, the super capacitor is used preferentially to involve in energy distribution during driving and braking, the super capacitor is used preferentially to conduct discharging during driving of a trolley bus, large electric power can be provided in a starting and accelerating stage, and power performance of the trolley bus is improved. During braking of the trolley bus, the super capacitor preferentially absorbs the feedback electric energy of motor braking, and electric consumption is reduced.