Abstract: The plastic deformation of weld interface under the welding pressure was investigated during the vacuum diffusion welding process of steel 45,and the effects of deformation rate on the welding quality and the process planning was discussed.The results show that the important factor affected on the quality of the vacuum diffusion welding process of steel 45 is the deformation rate under the fixed pressure.The temperature,pressure and holding time have relatively small influence on the welding quality when the deformation rate is beyond a critical value,which the whole welding interface achieves superposition completely by means of plastic deformation.The annealing treatment of post weld can improve the welding strength by changing the microstructures of the interface zone,and the holding time shortens.