In this paper, the laser surface remelting of ultra-high strength steel 300M was performed by using fiber lasers. The microstructure, hardness, and electrochemical corrosion behavior of the remelted surface at different laser powers and scanning rates were analyzed. The results show that the microstructures in the surface remelting zone primarily comprise martensite and retained austenite. The higher the scanning rate, the higher the microhardness. The average microhardness is about 704 HV. The electrochemical tests show that the self-corrosion potential of the surface of the material subsequent to laser remelting positively shifts, and the corrosion current density decreases. Thus, the corrosion resistance of the remelting layer is optimal when the laser power and scanning rate are 300 W and 33 mm/s, respectively.