Abstract: Experiments of electron beam welding was carried out for 56 mm thick Ti80 titanium alloy. Then the microstructure of the joint was studied, the results show that microstructure of weld metal is composed of needle martensite α' phase and β phase, and it's size increased by cooling speed's rising. The microstructure of HAZ can be divided into two regions. The microstructure of HAZ near fusion zone(Ⅰzone) was consisted of primary α phase,needle α phase, needle martensite α' and transferred β containing aciculate α; The microstructure of HAZ near the base metal(Ⅱ zone) was consisted of primary α phase, secondary α phase and β phase.