Abstract: The single-pass surfacing process test of robot vertical welding is carried out by using 10 mm-thick EH36 steel plate. The influences of wire feeding speed, welding speed, swing amplitude and residence time on the cross-sectional area of the weld are studied. The formula for calculating the cross-sectional area of the weld is modified by linear fitting, and the test result indicates that the modified cross-sectional area calculating formula can accurately predict the cross-sectional area of the weld bead by the vertical welding. By simplifying the cross-section shape of the weld bead and combining with the modified formula of the cross-sectional area of the weld bead, the height of the root pass weld bead, the number of welding seam layers, the number of weld layers, the lifting amount of the welding torch and the offset of the welding torch are deduced and calculated. The thick plate multi-layer and multi-pass path planning of the robot vertical welding is realized, and the research has certain reference significance for the automatic welding path planning of the thick plate by robot welding.