The invention belongs to the technical field of mechanical transmission and particularly relates to a serial gear shifting mechanism of a transfer case. A piston A, a piston B, a piston C and a piston D are disposed in the inner cavities of a cylinder body A and a cylinder body B. The piston C and the cylinder body A are sequentially located on the left of the cylinder body B and fixed with each other. The piston D is located on the right of the piston C and located in the cylinder body B. The piston A is located on the left of the piston C. The piston B is located on the left of the piston A. One end of a gear shifting shaft is fixed with the piston D, and the other end thereof is inserted into a cylinder cover and fixed with a conical sleeve. A signal switch is disposed on the cylinder cover. A shifting fork is located in the middle of the gear shifting shaft. A locking mechanism is disposed on the shifting block and in the middle of the cylinder cover. Inflating inlets I, II, III and IV respectively corresponding to the piston A, the piston B, the piston C and the piston D are formed in the cylinder body A. When inflation is performed respectively through the inflating inlets I, II, III and IV, the piston D is pushed to move by air pressure, the gear shifting shaft and the shifting fork are then driven to move, and the contact of the signal switch contacts with the conical sleeve to give out corresponding indications when corresponding gears are reached.