Fiber laser welding of low alloy high strength steel HC420LA was carried out with different laser power and defocusing distance. The effects of power and defocusing distance on the morphology, microstructure, hardness distribution and tensile properties of the joints were studied. The results show that the welded joints without porosity and crack can be obtained by fiber laser welding of the steel plate of HC420LA. When the laser welding power is 1 000 W, the weld formation is better. With the increase of laser power, the weld width increases, and the lower surface has a slight arc pit. The negative defocusingdistance has a significant effect on the weld section shape, and the ratio of upper and lower melting width increases with the increase of negative defocusingdistance.The main microstructure in the weld zone of laser welded HC420LA steel consists of martensite, ferrite and bainite. There is no softening zone in welded joint, and the hardness of weld is higher than that of heat affected zone and base metal. With the increase of power, the hardness of weld decreases gradually. The hardness of weld with larger negative defocusing distance is slightly higher than that with positive defocusing distance. In the direction perpendicular to the weld, the tensile strength of the welded joint is higher than that of the base metal. The strength-ductility balance of the weld increases slightly with the increase of power, and the change of the defocusing distance has a great influence on the strength-ductility balance, but there is no obvious rule.