Abstract: During the transverse welding of titanium alloy thick plates, the molten pool exhibits asymmetric flow within the narrow gap groove due to gravity, which affects weld formation. To improve weld quality, a self-developed method was used to achieve transverse asymmetric oscillation of the arc based on alternating magnetic field-assisted narrow gap argon tungsten-arc welding. Compared with conventional magnetic-controlled narrow gap welding technologies, the asymmetric oscillation of the arc generates different heat inputs on both sides of the narrow gap groove, resulting in different temperature gradients between the molten pool and the sidewalls within the groove, thereby actively controlling the asymmetric flow of the molten pool during transverse welding. The effects of excitation intensity and excitation frequency on weld formation were mainly investigated, and the flow behavior of the molten pool within the narrow gap groove was captured by a high-speed camera. Results indicate that the symmetry of the upper and lower sides of the weld and the sagging amount of the molten pool decrease gradually with the decrease of lower excitation intensity and reach optimal values when the upper and lower excitation intensities are 5 mT and 2 mT, respectively. However, the change in excitation frequency has no significant effect on the symmetry of the weld but mainly affects the contact angle of the weld, and good welded joints with upper and lower symmetry and a smaller contact angle are obtained at 0.5 Hz or 1 Hz. In addition, force analysis was conducted on the molten pool, and it is revealed that the forces acting on the molten pool at the upper sidewall and the lower sidewall are different; appropriately reducing the magnetic field intensity during the downward oscillation of the arc can reduce the force that promotes the downward sagging of the molten pool.