Laser welding of 200 μm thickness NiTiNb shape memory alloy is achieved by Nd:YAG pulse laser. The welding is annealed at 850 ℃. Microstructure and mechanical properties are analyzed by optical microscope, scanning electron microscope and precision tensile machine. The results indicate that a layered microstructure occurs before heat treatment. Grains in the fusion area are epitaxial growth and the weld center are full of small isometric crystals. The tensile strength of the welding joint reach 90% of the matrix. Small and light dimples are observed on the welding joint fracture, which means it is ductile fracture. All the isometric crystals in the welding joint vanish after heat treatment, and coarse grain region and segregated spot have alternate distribution. Stress induced martensite occurs in both matrix and welding joint, which is called pseudo-elasticity. The yield strength of the welding joint with heat treatment is only 60% of the matrix. The micro-hardness of the matrix and welding joint with heat treatment is lower than those of the unheated ones.