The invention relates to a 12-gear full-
helical-tooth double-intermediate-shaft all-
synchronizer gearbox which adopts a design of combination of a three-section closed structure shell and main and auxiliary gearboxes. The main gearbox is of a small-center-distance double-intermediate-shaft structure and gears adopt large-tooth wide 
helical teeth, so that the bearing capacities and the meshing stabilities of the gears are improved, the meshing 
noise is reduced and the bearing cavity of the main gearbox is improved. Due to the adoption of an input drop-and-insert structure, the gear speed ratio order difference is small and the fuel economy is high. Double-cone inertial type of synchronizers are respectively and additionally arranged at an inverted first gear and second and third gears, so that the friction moments of the synchronizers are increased, the gear shifting force is reduced and the 
impact and 
noise which are generated in the gear shifting process are reduced. Due to the adoption of a 
helical tooth 
planet gear system structure, the bearing capacities of the gears are improved; and a 
planet gear system is designed with the axial limit, so that the helical tooth 
axial force is offset inside. Due to the adoption of an installing form of an auxiliary gearbox 
synchronizer, the torque states of a 
planet carrier at the high and low gears in a certain range are optimized and the 
torsional strength of a planet carrier 
assembly is greatly improved. By the double-
support design of both the front and rear ends of the planet carrier, the influence of finished automobile factors on the normal work of the inside of the gearbox is reduced.