Using 2.5 mm+4 mm carbon steel material and ER50-G welding wire to conduct laser filling wire welding test of unequal thickness butt joint. The fatigue properties and macroscopic and microscopic appearance of the unequal thickness butt joints are studied,and the fracture characteristics are analyzed. The results show that the fatigue value of the join is 237.7 MPa when the specified life cycle is 1×10,7,. The fatigue fracture occurs at the welding toe on the side of the 2.5 mm plate. The fracture presents typical fatigue characteristics,and the fatigue lines are clear,and the fracture morphology of the transient area is a dimple of different sizes. This study provides a reliable technical support for laser fillet welding in the manufacture of carbon steel body.