The 3 mm thick 1Cr21Ni5Ti stainless steel lock butt joint was welded with fiber laser, the welding procedure characteristics are studied by analyzing the effect of different welding parameter on the joint quality,mechanical properties and microstructure. The results show that the main weld defects are porosity and weld pit in the weld,slow power reduction can reduce the depth of pit in weld,and negative defocus and high speed welding can effectively prevent the formation of porosity. The hardness of the weld decreases,along the weld center,the hardness of the middle area of the weld is the highest. Within a certain range of parameters,the mechanical properties of the joint are stable,and the average tensile strength of the weld reaches 816 MPa. The grain in the weld is equiaxed crystal,mainly is composed of ferrite and austenite. The amount of austenite in the middle of the weld is greater than that in the top and bottom of the weld,thus showing a higher hardness. Under the action of welding thermal cycle,the grain in the heat affected zone becomes relatively coarse,and is the weak area of weld.