A power transmission system for a vehicle, comprising: an engine unit; a transmission unit adapted to selectively be coupled with the engine unit and comprising: a transmission power input part adapted to selectively engage with the engine unit to transmit power generated by the engine unit, the transmission power input part comprising a first input shaft and a second input shaft coaxially fitted over the first input shaft; a dual clutch including an input terminal coupled with the engine unit, a first output terminal coupled with the first input shaft, and a second output terminal coupled with the second input shaft; two driving gears which are fixed on the first input shaft and the second input shaft respectively; a transmission power output part configured to output the power from the transmission power input part and comprising an output shaft and two driven gears; a first motor generator coupled with the transmission unit; and an output unit configured to transmit power output by the transmission power output part to wheels of the vehicle; characterized in that the output unit configured to rotate differentially relative to the output shaft, the two driven gears are fixed on the output shaft, and the two driven gears are meshed with the two driving gears on the input shafts respectively; and a synchronizer adapted to selectively synchronize the output unit and the transmission unit so as to drive the wheels of the vehicle via the power output by the output unit, wherein the synchronizer is disposed on the output shaft and adapted to selectively engage with the output unit. an engine unit; a transmission unit adapted to selectively be coupled with the engine unit and comprising: a transmission power input part adapted to selectively engage with the engine unit to transmit power generated by the engine unit, the transmission power input part comprising a first input shaft and a second input shaft coaxially fitted over the first input shaft; a dual clutch including an input terminal coupled with the engine unit, a first output terminal coupled with the first input shaft, and a second output terminal coupled with the second input shaft; two driving gears which are fixed on the first input shaft and the second input shaft respectively; a transmission power output part configured to output the power from the transmission power input part and comprising an output shaft and two driven gears; a first motor generator coupled with the transmission unit; and an output unit configured to transmit power output by the transmission power output part to wheels of the vehicle; characterized in that the output unit configured to rotate differentially relative to the output shaft, the two driven gears are fixed on the output shaft, and the two driven gears are meshed with the two driving gears on the input shafts respectively; and a synchronizer adapted to selectively synchronize the output unit and the transmission unit so as to drive the wheels of the vehicle via the power output by the output unit, wherein the synchronizer is disposed on the output shaft and adapted to selectively engage with the output unit. a transmission power input part adapted to selectively engage with the engine unit to transmit power generated by the engine unit, the transmission power input part comprising a first input shaft and a second input shaft coaxially fitted over the first input shaft; a dual clutch including an input terminal coupled with the engine unit, a first output terminal coupled with the first input shaft, and a second output terminal coupled with the second input shaft; two driving gears which are fixed on the first input shaft and the second input shaft respectively; a transmission power output part configured to output the power from the transmission power input part and comprising an output shaft and two driven gears; a first motor generator coupled with the transmission unit; and an output unit configured to transmit power output by the transmission power output part to wheels of the vehicle; characterized in that the output unit configured to rotate differentially relative to the output shaft, the two driven gears are fixed on the output shaft, and the two driven gears are meshed with the two driving gears on the input shafts respectively; and a synchronizer adapted to selectively synchronize the output unit and the transmission unit so as to drive the wheels of the vehicle via the power output by the output unit, wherein the synchronizer is disposed on the output shaft and adapted to selectively engage with the output unit.