In this paper, the longitudinal butt-welding experiment of 1 mm thickness 304 stainless steel pipe was carried out by laser self-fusion welding technology. Based on the control variable experiment of two factors and four levels, microstructure characteristics were tested by optical microscope, X-ray diffractometer and scanning electron microscope. Taking the surface forming and microhardness as the index, the optimum welding parameters were adopted with a laser power of 3.5 kW, welding speed of 30 mm/s, and the focal position of + 2 mm. Results show that the weld center is composed of fine equiaxed austenite and dendritic ferrite, and there are fine columnar dendrites at the edge of the weld, which grow along the direction perpendicular to the fusion line whose dimension is proportional to the welding heat input. Under the optimal welding parameters, the microhardness of heat affected zone is 221 HV, and the microhardness of weld center is 214 HV. There is a negative correlation between microhardness and heat input in the corresponding regions of different welded joints.