Laser forming repairing (LFR) technology is developed from the laser additive manufacturing, which has a high potential in high strength steel structures' repairing. 300M steel has been widely used in aviation and aerospace vehicles, to provide a high strength for aircraft landing gear and high strength bolts components, which in turn leads to a quick damage due to the severe service environment. If these damaged components can be repaired rapidly, the considerable savings in materials and costs can be achieved. In this work, the microstructure and mechanical properties of the LFRed 300M steel have been investigated. Results showed that the LFRed area can be clearly divided into three areas: the substrate zone (SZ), heat affected zone (HAZ) and repaired zone (RZ). The SZ was consisted of the mixture of martensite, bainite and a small amount of retained austenite. The HAZ presented an uneven martensite. The RZ presented an obvious heterogeneous microstructure, and the bainite, the mixture of martensite and bainite, and tempered martensite from the top to the bottom. After heat treatment, the microstructure became uniform with mixed tempered martensite and bainite. The tensile strength of the as-deposited LFRed 300M steel was far lower than those of the substrate. Its tensile strength and yield strength were 1459 MPa and 1163 MPa, respectively. After heat treatment, tensile strength (1965 MPa), yield strength (1653 MPa), elongation (11.7%) and reduction of area (38.4%) increased significantly and reached the same level of the substrate. Furthermore, compared to the as-deposited sample, the local strain of the RZ increased to 53% after heat treatment, and an obvious necking and breaking up happened as well. The strain hardening exponent of SZ and RZ were 0.1548 and 0.1138, which could be closely related to the compatible deformation capability.