Abstract: This invention relates to a main-box-and-auxiliary-box-structural twin-countershaft twelve-speed transmission, with a main box and an auxiliary box mainly. Said main box has an input shaft (1), a main shaft (10) and two countershafts (14) arranged symmetrically at the two sides of the main shaft, there being provided corresponding gear drive between the main shaft and the countershafts, there being provided six gears positioned independently and three synchronizers (4) on the main shaft, shift forks (5) for the synchronizers are connected to a shift lever via shift fork shafts (16) and via an operation mechanism.