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The laser butt welding of GH3625 superalloy fabricated by laser additive was conducted. The microstructure evolution and mechanical properties of the welded joints were analyzed. The results show that a great amount of Laves phase was only precipitated at the grain boundary of upper layer in HAZ, resulting in significant coarsening of the grain boundary. Along the upper layer to the lower layer of the weld, the fusion zone and the intermediate zone are gradually transformed from cell crystal, the columnar crystal and the equiaxed fine crystal of the upper layer to columnar crystal of the lower layer. The number of equiaxed fine crystal growing close to the fusion line is gradually increased. And the central zone is dendritic. The particle of fine γ′ phase is precipitated in the weld. A number of Laves phase precipitates in the inter-dendrtic of the intermediate zone, and the morphology gradually changes from the strip of the upper layer to the particle of the lower layer. The tensile strength of the welded joint is 872 MPa, which is up to 98.2% of the base metal. The elongation is up to 90.7% of the base metal, the fracture surface of weld joint is approximately 45° inclined plane, and the fracture mode is the mixed fracture mode of brittle and ductile.