Abstract: Objective This paper aims to address the problem of undercut defects that tend to occur during laser-arc hybrid horizontal welding, the influence mechanisms of laser-wire distance and laser power on molten pool stability and undercut defect formation in arc-leading mode are investigated. Methods Systematic experiments of laser-arc hybrid horizontal welding in arc-leading mode are conducted. The effects of laser-wire distance on weld formation, droplet transfer position and molten pool stability are analyzed. The improvement effect of laser power on molten pool instability is studied, and the interaction between laser-wire distance and laser power is revealed. Results An excessively small laser-wire distance causes droplets to directly impinge on the region behind the keyhole, inducing intense turbulence and resulting in intermittent undercut defects. An excessively large laser-wire distance weakens the thermal coupling effect, leading to insufficient filling of liquid metal in the molten pool and a significant increase in the occurrence probability of undercut defects. Appropriately increasing the laser power can significantly suppress molten pool surface fluctuations, improve the molten pool flow state, and reduce undercut defect formation. Under an appropriate laser-wire distance (3～5 mm) combined with a proper laser power, the most stable molten pool state and optimal weld formation quality can be achieved. Conclusion The interaction effect between laser-wire distance and laser power is revealed, and the optimal process parameter range for achieving molten pool stability and suppressing undercut defects is identified, providing a theoretical basis and process optimization guidance for the application of laser-arc hybrid welding technology in high-end manufacturing.