Based on miniaturized components’ characteristics, the method of assembling miniaturized gear and shaft together with corresponding calculating model of the interference amount are proposed. On the basis of main effecting factors analysis on the gear and shaft assembling interference amount, calculating formula including all factors effective on the interference amount necessary for reliable system running was built up. The methods of reverse calculating theoretical model was used to build up the equivalent simulation model of the theoretical one, together with simulation verification and case study for calculating formula. The results show that the cold assembling method is applicable for miniaturized gear and shaft, but in the assembling process,the interference amount compensating the shape error of contacting surfaces takes a large proportion, which is the main cause of stress variance on contacting surfaces.