The dissimilar metal welding of seal（ASS） and penetration assemblies（Inconel 690） has been investigated by vacuum EBW in advanced PWR nuclear power plant. After visual and radiographic inspections,it is found that the main defects of dissimilar metals welding between ASS and Inconel 690 are cold shuts and nail point. Sine the fast cooling rate and samples with thick wall,metal vapor cannot escape from the weld,resulting in cold shuts. As the working distance increases, both the weld penetration and the travel of metal vapor escape reduce,so that the size and number of cold shuts in weld decease. When the working distance matches with the welding heat input appropriately,the cold shut will disappear.