Abstract: In this paper, the TC4 titanium alloy of thin-walled parts were repaired by the Electron beam additive melting system, and the microstructure characteristics and microhardness distribution of the repaired workpiece was studied. The results showed that the accumulation’s macrostructure was composed of irregular β columnar grains which surrounded by αgrains, through the cross section. There’s also layered phenomenon appeared: the "bright" zone was irregular bulky lath martensite, and the "gray" zone was basket-weaved acicular martensite. Hardness decreases from the bottom of base metal to surface, and corresponding “bright” hardness arised obviously fluctuate.