The 0.1C-5Mn steels are welded by a fiber laser, and the effect of heat input on the microstructure, micro-hardness, tensile properties and formability of steel joints is investigated. The results show that the fusion zones (FZs) of joints under different heat inputs are always full of martensite structures, and in contrast the heat-affected zones (HAZs) are composed of fine martensite/bainite mixed structures, austenite and ferrite. The average micro-hardness of the FZs is always higher than that of the base materials. A slightly softened zone occurs in the subcritical HAZ when the heat input is 100 J·mm-1. All the tensile samples of welded joints fracture at the base materials and the fractures belong to the ductile ones. The ultimate tensile strength of joints is higher than that of base materials. The Erichsen cracks of welded joints initiate in the FZ and extend perpendicular to the weld towards the base material zone. The forming performance of tailor-welded blank parallel to the rolling direction is superior to that of the perpendicular.