This paper aims to explore the generation mechanism of blowhole defects in TiAl based alloys exhaust valve in the process of vacuum suction casting. By comparing the morphology and distribution of void defects in TiAl based alloys exhaust valve fabricated by different process parameters, the type of void defects is identified as blowhole. Combining with numerical simulation of calculating the fluid state of the alloy melt in the suction casting process, the reason of blowhole formation is determined. By optimizing the casting parameters in the process, blowhole is eliminated ultimately.