To address the severe traveling wave resonance caused by axial excitation resulting from system deformation in a lightweight thin-webbed gear under asymmetric support conditions, a finite element model was established that comprehensively considers casing stiffness and gear tooth meshing conditions. Through pre-stressed modal and harmonic response analyses, the traveling wave resonance characteristics within the operational speed range were investigated, and the influence of the gear’s structural parameters on this resonance was studied. Although spur gears generate no axial force under ideal conditions due to their tooth geometry, asymmetric arrangements can lead to misalignment caused by shaft deflection or bearing deformation. This misalignment decomposes the radial force into an axial component, exciting traveling wave resonance, with the axial vibration behavior primarily governed by the gear’s structural characteristics. The study reveals that thin-webbed spur gears are prone to structural vibration within the operational speed range, exhibiting high energy in low-order modes and lower energy in higher-order modes, making them susceptible to low-frequency traveling wave resonance. Furthermore, under lightweight constraints, increasing the thickness of the web and rim effectively raises the natural frequency and significantly reduces vibrational stress, albeit at the cost of added weight. In contrast, increasing the web offset distance reduces the support stiffness, aggravating vibrational stress.