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As oxidizable metals, titanium alloys are very active on melting point. It is easier for large-size titanium castings to react with the casting materials which will affect the surface quality of the castings during casting . In this experiment, large-scale titanium alloy castings were casted by Yttrium oxide mould in vacuum consumable shell furnace, and the surface layer of samples with different thickness was tested and analyzed by SEM, XRD and EDS. The results show that there is a certain thickness of oxygen diffusion layer on the surface of the samples. And there is a certain thickness of tissue transition zone on the surface. Finally, the experiment was carried out by acid. Finally, the acid pickling method was used to remove the reaction layer and the transition zone of the sample surface, which effectively improved the surface quality of the castings.