When the heat input of the manganese steel in laser welding is 150500 J/mm, the weld forming is better. When the heat input is small, the weld microstructure is martensite, and with the increase of heat input, a small amount of bainite is generated in the weld. The prior-austenite grain boundary, martensitic packet boundary, and block boundary in the weld microstructure of the medium manganese steel can hinder crack propagation. The crack propagation energy in the weld zone is inversely proportional to the average effective grain size (>15° high-angle grain boundary) and positively proportional to the high-angle grain boundary density. The impact toughness of the weld at -40 ℃ increases first and then decreases with the increase of heat input. When the welding heat input is 150 J/mm, the dislocation group and the twin martensite structure reduce the crack initiation energy and make it easy to crack, and the fracture is a cleavage fracture. When the heat input is 300 J/mm, the effective grain size decreases, the high-angle grain boundary density is the highest, the crack initiation and propagation energies increase, and the fracture is a ductile fracture. When the heat input is greater than 300 J/mm, the effective grain size increases, the high-angle grain boundary density decreases, and the crack propagation energy decreases. When the heat input is 500 J/mm, the high-angle grain boundary density is the lowest, the crack propagation energy decreases sharply, and the fracture is a cleavage fracture.