The invention provides a transmission outer-hub piston press-mounting guide device. The transmission outer-hub piston press-mounting guide device comprises a piston guiding sleeve and a positioning column which are connected coaxially, wherein the outer surface of the piston guiding sleeve is in butt joint with a piston, the positioning column penetrates through an outer hub of an automobile transmission and is plug-connected to a bearing platform of a press-mounting device, the piston guiding sleeve comprises an outer hub positioning block embedded into the inner surface of the edge of the piston guiding sleeve and a fixing screw, the outer hub positioning block comprises a positioning piece, two protrusions are arranged on the edge of the positioning piece and are plugged into the positions corresponding to the outer hub of the automobile transmission for positioning, screw holes are formed in the piston guiding sleeve, the positioning piece and the outer hub of the automobile transmission, and the piston guiding sleeve, the outer hub positioning piece and the outer hub of the automobile transmission are connected through the fixing screw. Compared with the prior art, the transmission outer-hub piston press-mounting guide device is simple in structure, low in manufacturing cost, high in safety, capable of saving time and convenient to operate and has very strong specialization.