Abstract: Using the 2.5mm and 4.0mm plate thickness high weathering steel Q350EWR1 hand-held laser wire welding butt joint as the research object, the microstructure of each area of the joint was observed and the mechanical properties were tested to analyze the influence of plate thickness on the microstructure and mechanical properties of the joint. The results show that the base material is composed of ferrite and pearlite. In the heat affected zone, there is only normal fire zone without coarse crystal zone. The weld is composed of proeutectoid ferrite and bainite. The hardness distribution of the two joints is in the shape of "mountain". Due to the changes in the microstructure and grain size of the weld and heat affected zone under the action of welding heat cycling, the relationship between the hardness size of the joint regions is as follows: weld zone > heat affected zone > base material. Due to the difference in pearlite and ferrite content in the base material area, the tensile strength of the 4.0mm butt joint is lower than that of the 2.5mm butt joint, and the tensile strength is 491 MPa and 558 MPa respectively, both of which break at the base material. Under the specified cycle life, the fatigue strength of the 2.5mm butt joint is lower than the fatigue strength of the 4.0mm butt joint.