Abstract: In order to suppress the welding crack of TiAl alloy for aero engines, the electron beam welding test on TNM (Ti-43.5Al-4Nb-1Mo-0.1B) alloy test plate was carried out at 300℃ and 500℃ under overall preheating. The microstructure of the joint was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD), and the effects of preheating temperature on the microstructure such as cracks, weld morphology, grain size, phase composition and hardness were analyzed. The results show that the weld width increases with the preheating temperature, and the crack disappears when the preheating temperature is 500℃. Grain coarsening in the near-heat affected zone and refinement in the far-heat affected zone were observed. With increasing preheating temperature, the content of α₂ phase decreased and the content of γ phase increased in the welded joint, γ phase became the main phase under the preheating temperature of 500℃. In addition, the nearly lamellar feature of α₂ phase disappears under the preheating temperature of 500℃. With increasing preheating temperature, the dislocation density and hardness of the weld are obviously reduced.