Abstract: Laser-arc hybrid welding technology has been widely applied in automotive, rail transportation, and shipbuilding industries due to its high efficiency and superior quality. However, undercut defects significantly limit the further promotion of this technology. This paper systematically analyzes influencing factors, formation mechanisms and control methods of undercut defects in laser-arc hybrid welding. Research indicates that undercut formation is comprehensively influenced by welding parameters and welding conditions, primarily including laser and arc parameters, heat source coupling effects, shielding gas composition, as well as base material properties, groove geometry and surface conditions. From the perspective of formation mechanisms, undercut defects can be attributed to two mechanisms. First is force imbalance in the weld pool, manifested as deviation of the weld pool force state from the equilibrium required for adequate toe filling during solidification, resulting in regularly distributed shallow and wide undercut. Second is dynamic instability of weld pool, originating from transient disturbances (such as droplet impact, arc fluctuation, and keyhole collapse) that induces weld pool flow fluctuations, preventing stable toe filling and leading to irregularly distributed narrow undercut with inconsistent severity. Regarding control methods, welding parameter optimization and equipment improvement are currently the primary approaches, including heat source spacing adjustment, power ratio matching, shielding gas optimization, and techniques such as oscillating laser and external magnetic fields. Despite significant progress, dynamic response mechanisms of multi-physics field coupling still require in-depth investigation. Future research needs to develop comprehensive prediction models for undercut defect to provide theoretical guidance for the industrial application of this technology.