12Cr12Mo stainless steel is an important material for static blade ring of steam turbine. The heterogeneous microstructure and properties of welded joints of thick 12Cr12Mo stainless steel are often occurred. In this paper, 12Cr12Mo butt joint of thick plate was prepared by electronic welding. The microstructure and mechanical properties of the joint were studied. The results show that the inhomogeneity of microstructure and mechanical properties along with thickness are found. The fusion zone is mainly quenched martensite, the heat affected zone is the mixed microstructure of martensite and tempered sortensite, and the microstructure of base material is tempered sortensite. The grain size of the fusion zone decreases with the increase of the penetration depth while the grain size of the fusion zone decreases with the increase of the penetration depth. The grain size of the heat affected zone at different thickness positions is the same. However, the grain sizes of the heat affected zone are smaller than that of the fusion zone and the base metal. The tensile strength and hardness increase gradually as the penetration depth increase while the elongation decreases continuously. The fracture location of the layered tensile specimens appears in the fusion zone, and the fracture dimple becomes smaller and shallower with the increase of penetration depth. The difference of microstructure evolution caused by heat input and cooling rate along with thickness of the weld is the main reason for the inhomogeneity of microstructure and mechanical properties.