For improving the understanding of laser-metal inert gas (MIG) hybrid welding, the effects of laser power, arc current and groove gap on critical speed of this process are studied on Q235 mild steel with thickness of 4 mm. The results show laser power and arc current are direct proportional to the critical speed of hybrid welding. The maximum and minimum critical speed, v_max and v_min are mainly determined by the laser power and arc current, respectively. Increasing groove gap can both increase v_max and v_min but the effect on v_min is bigger. In the experiment, the effects of groove gap on v_max and v_min can reach 20% and 35% respectively. Moreover, the reliable welding speed range of hybrid welding, Δv is increased with laser power. For the constant laser power, increasing arc current will decrease Δv.