Butt welding of 0.2mm thick TiNi shape memory alloy sheet was successfully realized by using micro impulse laser whose average power is 80W and the microstructure of welded joint was study in this paper. The results show that, the welded joint of micro laser welding can be divided into four zones according to grain size and microstructure. The microstructure in base metal zone is rolled structure and the grains are coarse and heterogeneous. The microstructures of welded seam center zone are fine equiaxed crystals and the microstructures of both lower surface and upper surface edge zone are columnar crystals. There is almost no obvious coarse grain heat-affected zone at the edge between welded seam and base metal. There is obvious segregation layer in local area of welded seam because the content of Ti and Ni elements is changed and different with base metal during the crystallizing course of welding pool.