Wire arc additive manufacturing technology is widely used in aerospace, weapon equipment and other fields due to its advantages of short manufacturing cycle and no size limitation of manufacturing parts. The basis of wire arc additive manufacturing technology to realize the direct forming of complex metal parts is that the morphology of the welding bead has uniform and stable repeatability in the deposition process. However, wire arc additive manufacturing is a process involving multiple physical fields such as electric field and magnetic field coupling. When the deposition behavior of metal droplets occurs on non-horizontal surface, the molten pool is prone to destabilizing flow under gravity induction, resulting in asymmetric weld bead topography, reducing the forming accuracy of parts, and even leading to forming defects. Based on this, the induction mechanism and morphology evolution mechanism of the asymmetric molten pool in the wire arc additive manufacturing process were studied by combining experiments and simulations, and the forming characteristics of the weld bead was explored. It was found that with increasing plate angle, the deflection of the weld pass increased gradually (0, 0.454 9, 1.937 4, 2.331 9 mm), the depth of the molten pool decreased gradually(1.935, 1.763, 1.684, 1.576 mm), and the degree of asymmetry of the formed weld pass increased gradually. In addition, the evolution law of the asymmetric molten pool morphology during the deposition process was studied by means of experiments and numerical simulations, and it was revealed that the asymmetric flow of surface metal fluid induced by gravity is the cause of the deterioration of the weld bead morphology. This study provides useful guidance for the subsequent research on the suppression of asymmetric pass morphology and the high quality forming of three-dimensional complex metal components.