The invention discloses a heavy 12-gear AMT transmission assembly which comprises a transmission case, a main box, an auxiliary box and an automatic gear shifting control device; the automatic gear shifting control device comprises a TCU, a gear selecting and shifting actuator, a main box intermediate axle brake control valve, a main box intermediate axle brake, an auxiliary box control valve and an auxiliary box air cylinder, where in the gear selecting and shifting actuator, the main box intermediate axle brake control valve and the auxiliary box control valve are all connected with the TCU, and the main box intermediate axle brake comprises a friction plate and a piston; when the main box shifts gears, the main box intermediate axle brake control valve receives a control signal from the TCU, controls the piston to act through an air channel, the piston presses the friction plate tightly, braking on a main box intermediate axle is achieved, and then the gear selecting and shifting actuator completes a gear shifting action of the main box. Accordingly, a main-auxiliary box double-intermediate axle structure is adopted, the bearing capacity is high, the input torque reaches 2,300 Nm, the life reaches 1500,000 kilometers, the speed ratios of all the gears are reasonable, the speed ratio of the first gear is high, and the reliability is high.