In order to realize the development and optimization of thick plate welding process for offshore wind power towers, the applicability of high-power laser welding technology in thick plate welding was explored based on laser blunt edge bottoming welding and narrow gap laser wire filling welding. The high-efficiency welding of the Q355ND low-alloy high-strength steel plate was completed, and the excellent welded joint of the 25 mm thick plate was obtained. The welding seam was mainly completed by 10 mm laser primer welding and 2 laser wire filling welding. The stacking height of each layer of filler wire welding were 7.2 mm and 7.8 mm respectively, and the width of the welding seam was about 3.8 mm. Optical microscope, microhardness tester and electrochemical workstation were used to analyze the microstructure, microhardness and corrosion resistance of the weld center, heat affected zone and base metal of laser backing welding, and narrow gap laser wire filler welding. The results show that the columnar grain structure is ferrite and a small amount of martensite in the center of the laser backing welding weld, and the microstructure of the heat affected zone is granular bainite and martensite. The microstructure is composed of acicular and massive ferrite, with a small amount of bainite and massive ferrite interphase distribution. The structure of the heat affected zone is lath martensite and bainite. The average hardness of the center of the backing weld is 360.5 HV, and the average hardness of the heat-affected zone of the wire filler weld is 349.1 HV. The corrosion resistance and pitting corrosion resistance of the weld area of the welded joint are excellent and higher than that of the base metal.