Abstract: High-temperature structural material TiAl based alloy has poor machinability. It is challenging to fabricate complex structures. And the processing cost is relatively high. GTA arc additive manufacturing is a promising technology to synthesis TiAl based alloy in situ with increased flexibility and low cost. However, crack control is still essential during fabrication. Preheating is effective in avoiding cracking during additive manufacturing. In this work, the Ti6Al4V and ER1100 wires are used as feedstock to fabricate TiAl based alloy in situ with an aluminum content of 50 at%. Preheating temperatures of 200, 300, 450 ℃ are used to investigate the influences on the microstructures and mechanical properties of the fabricated specimen. Results show that, with the preheating temperature increasing, more bulk γ phases precipitate on the boundaries of γ/α₂ lamellar clusters in the top and middle region of the specimen, while the influences on the microstructures at the bottom are not significant. The proportion of α₂ decreases with the preheating temperature, leading to a diminishing hardness. At the same time, the percentage of bulk-like γ phases increases, which results in the promotion of compressive properties. When the preheating temperature is set at 450 ℃, the fabricated specimen shows the highest compressive strength and compression ratio.