Abstract: Through the analysis of surface cracks of ZTA7 titanium investment casting,the causes of surface cracks on titanium alloy casting without the influence of surface α contamination layer were determined. The results show that when the ZTA7 casting is repair welded, the microstructure of the substrate is changed and needle α phase appears due to the thermal effect during repair welding, the oxygen diffuses to titanium alloy and oxygen-rich diffusion layer is formed.Therefore, the plastic toughness of the heat-affected zone is greatly reduced, and under the action of welding stress, cold welding cracks are formed.