Abstract: The characteristics and arrangement of heat sources are critical factors influencing the welding process and weld quality. In this study, hybrid welding experiments on low-carbon steel were conducted using a high-power fiber laser and a direct current TIG arc. By systematically adjusting the laser power, arc current, welding speed, and heat source arrangement(laser-leading and arc-leading), the effects of thermal characteristics on the welding process were investigated. Emphasis was placed on analyzing the correlation among hybrid plasma behavior, weld surface formation, keyhole stability, and porosity defects. The results indicate that the size of the hybrid welding plasma increases with rising laser power and arc current. Under different heat source arrangements, the surface roughness and profile height of the weld generally decrease with increasing welding speed. Under identical process parameters, the weld profile height in the laser-leading mode is significantly greater than that in the arc-leading mode. When the laser-to-arc energy ratio approaches 1 ∶1, keyhole-mode porosity tends to form inside the weld, whereas under laser-dominant or arc-dominant conditions, the number of pores decreases markedly. The influence of the laser-to-arc energy ratio in the hybrid heat source on pore generation is less significant. Further analysis reveals that the arc force acting on the protruding liquid column near the keyhole opening is the primary cause of differences in weld surface formation under different heat source leading modes. Meanwhile, the arc force drives the molten liquid near the keyhole opening to block it, leading to keyhole instability and collapse, which is the main mechanism for the formation of keyhole-mode porosity. This study provides important insights for a deeper understanding of the hybrid welding process mechanism and the control of weld defects.