The Al-Cu-Li alloy is welded by using the vacuum electron beam welding (EBW). The microstructure of joint under the as-welded (AW) condition and the effect of post-weld heat treatment (PWHT) on the microstructure of joint are investigated. The results show that under the AW condition, the microstructure of weldment is a typical dendrite, and there are a lot of eutectic structures which are mainly composed of α+θ′（Al2Cu） phase distributed in the dendrite boundary. The quantity of strengthening phase is less in the weld center. The microstructure of joint obviouslychanges after PWHT. The dendritic structure in the center of weldment is transformed into equiaxed grains. The segregation phenomenon of grain boundary is eliminated under the AW condition. Many strengthening phases precipitate in the weldment,such as spherical δ′(Al3Li) phase and fine needle-like T1(Al2CuLi) phase, and the tensile strength of welded joint is greatly improved. The tensile strength of joint is enhanced from 348 MPa under the AW condition to 423 MPa after PWHT, and the tensile fracture of joint presents the characteristic of ductile fracture.