The porosity is an important kind of weld defects in deep-penetration laser-arc welds. This paper gives an Experimental study on relationship between molten pool behavior and keyhole-induced porosity in pulsed laser-arc hybrid welding. In this research, the impact effect of laser pulse on the weld pool, and live behavior of the laser keyhole and the porosities in the weld were investigated in detail. Results show that the impact effects from the laser pulse depend on the laser pulse parameters and stronger impact effect helps restrain the porosity formation. The laser keyhole state dominants the welding pool behavior. The keyhole dimension and open-close state directly influences the porosity formation. Extremely high laser pulse power always brings about porosities after welding. Lengthening the laser keyhole opening time and giving the gas enough time to escape will be an effectively way to restrain the porosity.