Abstract: Objective In order to meet requirements of special structural position of ship design, influence of oscillating parameters (oscillating frequency and oscillating amplitude) on formation and porosity of single pass was studied. Methods 20 mm thick 10Ni5CrMoV high strength steel plate was used as test material, narrow gap oscillating laser welding with filler wire was used for horizontal welding. Formation and porosity of weld were measured, and laser energy distribution and spatial distribution state of laser in groove were measured. Results The results showed that, compared with conventional laser welding, oscillating laser welding could improve single pass formation of 10Ni5CrMoV steel and effectively inhibit formation of porosity during horizontal welding process. Oscillating amplitude was the main factor affecting surface formation, which affected distribution state of laser energy in groove and thus affected surface formation of single pass. In this test, when oscillating amplitude was 2.2 mm, the formation was the best. Oscillating frequency was the main factor affecting formation of porosity, and oscillating frequency greater than 150 Hz could avoid formation of porosity in this test. Conclusion The main reason for oscillating laser to suppress porosity defects in the process of narrow gap laser horizontal welding was that peak energy distribution decreased due to the increase of frequency, bubble escape path increased due to the increase of weld width, and decrease of penetration made escape distance decrease. Therefore, pass with a small penetration-width ratio could effectively avoid formation of porosity. Finally, reasonable oscillating amplitude and oscillating frequency were used to realize narrow gap laser welding of 20 mm thick high strength steel.