Electron beam welding (EBW) will be used in the manufacture of the Chinese Fusion Engineering Testing Reactor (CFETR) vacuum vessel (VV). The VV is the main support structure of a fusion reactor, and it has strict requirements on welding quality. To improve the properties of welded joints, EBW with beam oscillation was investigated on 316L austenitic stainless steel with 50 mm thickness. Microstructure and hardness were tested with optical microscope and microhardness tester. The results showed that EBW with beam oscillation improved the weld morphology and uniformity of microstructure. The weld microstructure was composed of austenite and ferrite. The morphology of the ferrite was gradually changed from lathy/skeletal grains in the top layers of the joints to equiaxed grains in the bottom layers. Also, beam oscillation affected the solidification mode of the fusion zone (FZ). At the bottom of the weld with beam oscillation, the solidification mode changed from primary ferrite with second-phase austenite (FA) to primary austenite with the second-phase ferrite (AF). However, microhardness tests demonstrated that beam oscillation had less impact for microhardness. The microhardness of the FZ center line increased along the thickness.