Abstract: CoCrNiSi_x (x=0, 0.1, 0.2) medium entropy alloys were butt welded by electron beam welding. The microstructure observation, X-ray diffraction analysis (XRD), energy dispersive spectroscopy (EDS) analysis, microhardness test and tensile test of the welded joints were carried out. The effects of Si content on the microstructure and properties of CoCrNiSi_x medium entropy alloy welded joint were compared and analyzed. It is found that there is no obvious heat affected zone in the welded joint of CoCrNiSi_x medium entropy alloy, and the microstructure of both sides of the fusion zone presents two different morphologies.Compared with the base metal, the weld zone has obvious grain refinement.From the base metal to the weld zone, each element is evenly distributed and Si element is mainly distributed in the crystal. The microhardness of the weld zone is obviously higher than that of base metal.The fracture position of tensile samples of all welded joints is at the base metal.The fracture form is ductile fracture and the fracture morphology is mainly dimple.With the increase of Si content, the yield strength and tensile strength also increase.