A transmission unit for a vehicle, comprising: a plurality of input shafts (11, 12), each of the input shafts (11, 12) being provided with a shift driving gear (1a-5a) thereon; a plurality of output shafts (21, 22), each of the output shafts (21, 22) being provided with a shift driven gear (1b-5b) configured to mesh with a corresponding shift driving gear (1a-5a); a motor power shaft; a first motor gear freely fitted over the motor power shaft and configured to rotate together with one of the input shafts (11, 12); a second motor gear freely fitted over the motor power shaft and configured to rotate together with one of the output shafts (21, 22); a motor synchronizer disposed on the motor power shaft and between the first and second motor gears (31, 32); a reverse gear freely fitted over the motor power shaft; a middle idler; a reverse idler gear configured to mesh with the reverse gear and to selectively rotate together with the middle idler; and a reverse synchronizer configured to synchronize the middle idler and the reverse idler gear, wherein the plurality of input shafts (11, 12) comprise a first input shaft and a second input shaft coaxially fitted over the first input shaft; wherein the plurality of output shafts (21, 22) comprise first and second output shafts (21, 22) disposed parallel to the first and second input shafts (11, 12); wherein the plurality of shift driving gears (la-5a) comprise: a first-gear driving gear disposed on the first input shaft, a second-gear driving gear disposed on the second input shaft, a third-gear driving gear disposed on the first input shaft, a fourth-gear driving gear disposed on the second input shaft, and a fifth-gear driving gear disposed on the first input shaft; wherein the plurality of shift driven gears (lb-5b) comprise first-gear to fourth-gear driven gears (1b-5b) disposed on the first output shaft, and a fifth-gear driven gear disposed on the second output shaft; wherein the power transmission system further comprise a first-third gear synchronizer disposed between the first-gear driven gear and the third-gear driven gear, a second-fourth gear synchronizer disposed between the second-gear driven gear and the fourth-gear driven gear, and a fifth gear synchronizer disposed on a side of the fifth-gear driven gear; and wherein the reverse idler gear is freely fitted over the second output shaft, and the middle idler is configured to mesh with the second-gear driving gear, characterized in that a transmission gear is fixed on the second output shaft and meshed with the second motor gear, and in that the reverse synchronizer and the fifth gear synchronizer share a shift fork mechanism. a plurality of input shafts (11, 12), each of the input shafts (11, 12) being provided with a shift driving gear (1a-5a) thereon; a plurality of output shafts (21, 22), each of the output shafts (21, 22) being provided with a shift driven gear (1b-5b) configured to mesh with a corresponding shift driving gear (1a-5a); a motor power shaft; a first motor gear freely fitted over the motor power shaft and configured to rotate together with one of the input shafts (11, 12); a second motor gear freely fitted over the motor power shaft and configured to rotate together with one of the output shafts (21, 22); a motor synchronizer disposed on the motor power shaft and between the first and second motor gears (31, 32); a reverse gear freely fitted over the motor power shaft; a middle idler; a reverse idler gear configured to mesh with the reverse gear and to selectively rotate together with the middle idler; and a reverse synchronizer configured to synchronize the middle idler and the reverse idler gear, wherein the plurality of input shafts (11, 12) comprise a first input shaft and a second input shaft coaxially fitted over the first input shaft; wherein the plurality of output shafts (21, 22) comprise first and second output shafts (21, 22) disposed parallel to the first and second input shafts (11, 12); wherein the plurality of shift driving gears (la-5a) comprise: a first-gear driving gear disposed on the first input shaft, a second-gear driving gear disposed on the second input shaft, a third-gear driving gear disposed on the first input shaft, a fourth-gear driving gear disposed on the second input shaft, and a fifth-gear driving gear disposed on the first input shaft; wherein the plurality of shift driven gears (lb-5b) comprise first-gear to fourth-gear driven gears (1b-5b) disposed on the first output shaft, and a fifth-gear driven gear disposed on the second output shaft; wherein the power transmission system further comprise a first-third gear synchronizer disposed between the first-gear driven gear and the third-gear driven gear, a second-fourth gear synchronizer disposed between the second-gear driven gear and the fourth-gear driven gear, and a fifth gear synchronizer disposed on a side of the fifth-gear driven gear; and wherein the reverse idler gear is freely fitted over the second output shaft, and the middle idler is configured to mesh with the second-gear driving gear, characterized in that a transmission gear is fixed on the second output shaft and meshed with the second motor gear, and in that the reverse synchronizer and the fifth gear synchronizer share a shift fork mechanism.