In order to study the microstructure and mechanical properties of laser welded joint of 300M low alloy ultra-high strength steel, fiber laser is used to perform the self-fusion welding butt test of 300M steel. The weld morphology, microstructure change, and mechanical properties of welded joints are studied by means of tensile test and other analytical methods. The rapid solidification process of weld metal is discussed. The results show that the weld cross-section of the laser welded joint of the 300M low alloy ultra-high strength steel has a typical "Y" shape. The weld is mainly composed of columnar dendrites, and the weld structure is mainly composed of martensite and retained austenite. The heat effect near the weld is divided into a hardened zone and a softened zone. The hardness of the hardened zone is about 734 HV, the hardness of the softened zone is about 456 HV, the fracture location of the joint is the softened zone, and the tensile strength of the joint is 1269 MPa, which is about the 65% of the strength of host material.