The invention relates to a device for assembly of a first shaft and a bearing of an automobile gear box, in particular to a press-fitting mechanism for providing a reactive force by static friction. A pres-fitting hydraulic cylinder is arranged on the top of a mechanism body; the top of a lifting frame is connected to an output end of the pres-fitting hydraulic cylinder, and the bottom of the lifting frame is provided with a press head; a telescopic hydraulic cylinder is arranged on the bottom of the mechanism body, and an output end of the telescopic hydraulic cylinder is connected to a shrinkable sleeve positioned between the pres-fitting hydraulic cylinder and the press head; the shrinkable sleeve includes a shrinkable cone and a cone base, the shrinkable cone is arranged on the bottom of the mechanism body, and the cone base is connected to the output end of the telescopic hydraulic cylinder and is driven to lift by the telescopic hydraulic cylinder; a first shaft is inserted in the shrinkable cone, and the shrinkable cone is shrank through the cone base which is driven to lift by the telescopic hydraulic cylinder and is axially positioned; a press-fitting force of the press head for pressing the bearing counteracts a static friction force between the shrinkable cone and the first shaft. The press-fitting mechanism meets an assembly technique demand, improves the assembly accuracy of the first shaft and the bearing, reduces the labor intensity of an operation worker, and improves the assembly efficiency of the first shaft.