Tensile strength 700 MPa Nb-Ti microalloyed C-Mn steel is welded using high power fiber laser. The effects of heat input (162、175、187、226 J/mm2) on microstructure, tensile properties, microhardness, impact toughness are studied. The results show that the microstructure of welded seam (WS) and coarse grain heat affected zone (CG-HAZ) are mainly lath martensite, and a little of bainite and ferrite. The original austenite grain size of CGHAZincreases with heat input increasing. The effects of heat input on microstructure of fine grain heat affected zone (FG-HAZ) and mixed grain heat affected zone (MG-HAZ) are not obvious. The average hardness of WS and HAZ are greater than that of base material (BM), and the hardness of WS and peak hardness of weld joint decrease with the heat input increase because of the formation of ferrite and bainite with the increase of heat input. The tensile strength of welded joint are higher than that of BM at four different heat inputs. The impact toughness of weld joint is better than that of BM at four different heat inputs, and both weld joint and BM fracture mode are ductile fracture. The effect of heat input on weld joint impact energy is not obvious.