Abstract: 1.5 mm thick TRIP590 steel plates were butt welded by fiber lasers. Then the macro-morphology and microstructure of the joint was observed, the hardness and tensile properties were tested, and the influences of welding speed (0.050, 0.067, 0.083 m ·s^-1) on the weld seam formation and on the microstructure and mechanical properties of welded joint were analyzed. The results show that a relatively low and high welding speed was not good for the formation of weld seam and led to relatively large depressions on weld seam surface. The welding speed had little influence on the microstructure and properties. The microstructures of weld seam and heat affected zone near weld mainly consisted of martensite, resulting in relatively high hardness of the two zones, which was nearly twice as high as that of base metal. The microstructure of heat affected zone near base metal was mainly composed of martensite and ferrite, and the martensite amounts decreased gradually with the increase of distance to the weld center, leading to the sharp decrease of hardness. Both the yield strength and tensile strength of the joints were slightly higher while the plasticity was slightly lower than those of base metal. The joints fractured in base metal and showed a ductile fracture.