Abstract: QCr0.8/TC4 electron beam welding seam was composed of fusion zone and reaction layer, and the structure, phase composition and extent of reaction played important part in the joint tensile strength.According to the Fick diffusion law, the ratio of elements Cu and Ti diffusion flux can be calculated.In the reaction layer, CuTi was generated firstly, which was continuous and distributed in the reaction layer.The energy spectrum analysis indicated that the reaction layer composition was Cu +Cu_xTi zone and CuTi based solid solution zone.And CuxTi was the mixture of several compounds, such as Cu₄Ti, Cu₃Ti, and Cu₂Ti and so on.Due to the rapid cooling, The zone near TC4 fusion line couldn't generate the second phase compound in the weld.Therefore, the continuous intermetallic compound CuTi rusulted in high residual stress and hard brittle layer, which was the main factor effecting the joint mechanical properties.