The laser-arc hybrid welding of HG785D high-strength steels is performed and the effects of laser power and wire feeding speed on the hybrid welding process under different heat source orders are investigated. The dynamic behavior of plasma in the hybrid welding process is investigated. The electron temperature and electron density in plasma under different heat source orders are obtained and the coupling mechanism of heat sources in the welding process is disclosed. The results show that, the weld penetration decreases first and then increases with the increase of laser power, and it decreases gradually with the increase of wire feeding speed. Under the laser-leading condition, the plasma possesses a large volume and a high electron temperature, and the weld penetration is large, simultaneously, the tensile strength of joints is high but the plasticity is weak, and the micro-hardness at each area of joints is high, if compared with those under the arc-leading condition.