Abstract: Ultra-high strength AerMet 100 Steel was prepared by laser direct deposition. The microstructure and mechanical properties of the alloy were studied by optical microscope, scanning electron microscope, electron probe, microhardness tester, tensile testing at ambient temperature and impact test. The results show that The energy of laser forming line suitable for AerMet100 high strength steel is 175 ~ 210 J/mm. Considering the forming efficiency and fine crystal effect, the process parameters with high forming speed should be preferred.The deposited structure is columnar cellular crystal along the solidification direction, which is mainly composed of the lamellar martensite and residual austenite at the cellular dendrite boundary. The lamellar martensite is mainly due to the rapid cooling during laser forming process, while the residual austenite is mainly due to the segregation of Cr, Mo and Ni elements. The hardness of the deposition is similar to the substrate, but there is obvious softening of the heat affected zone in the deposition direction, which is mainly due to the heat accumulation in the deposition process to promote the tempering of tempered martensite in the matrix at high temperature. The superior mechanical properties of AerMet100 high strength steel were obtained by laser deposition when P = 1700 W, Vs = 10 mm/s. The tensile strength and yield strength reached 1865.3 MPa and 1585.5 MPa, respectively, and the elongation after breaking reached 12.4%. With the decrease of linear energy density, the shear lip of tensile fracture disappears and the depth of dimple becomes shallow. The impact fracture surface increases and the ductile fracture changes to brittle fracture.