Abstract: Laser butt welding was carried out on 800 MPa microalloyed carbon manganese steel plates under different heat inputs (1.27, 1.52, 1.90 kJ·cm^-1), and then the effects of heat input on microstructure, room temperature tensile property and -40℃ impact property of the welded joints were studied. The results show that the heat input had little effect on microstructure of the welded joint. Under three heat inputs, the microstructures of the weld zone and coarse-grained heat affected zone consisted of lath martensite, that of the fine-grained heat affected zone was composed of fine grain ferrite and martensite-austenite (M-A) constituent at grain boundaries, and that of the mixed grain zone was composed of ferrite and M-A constituent with different size. With increasing heat input, the grain size of coarse-grained heat affected zone increased and that of the fine-grained zone changed little. The heat input had little effect on the tensile properties. All tensile fracture positions were located in base metal. With increasing heat input, the impact energy of the weld zone first increased and then decreased, and reached the maximum value when the heat input was 1.52 kJ·cm^-1. The impact fracture exibited ductile fracture.