The invention discloses low-yield ratio and high-strength steel for bridges and a manufacturing method thereof. The steel comprises the following chemical components by weight percent : 0.06-0.10 percent of C, 0.20-0.45 percent of Si, 1.20-1.50 percent of Mn, no more than 0.010 percent of P, no more than 0.0020 percent of S, 0.30-.060 percent of Ni, 0.20-0.50 percent of Cu, 0.15-0.50 percent of Mo, 0.025-0.060 percent of Nb, no more than 0.035 percent of Ti, 0.020-0.040 percent of Alt, and Fe and unavoidable impurities in balancing amount. According to the invention, through smelting, refining, performing a reasonable thermo-mechanical control process (TMCP) and optimizing tempering heat treatment production, the tensile strength of the steel is 700-780 MPa, the yield strength is 530-600MPa, the extensibility is greater than or equal to 18 percent, the yield ratio is less than or equal to 0.80, and 40 below zero DEG C low-temperature longitudinal impact is greater than or equal to 100 J, so that the steel is good in obdurability matching and low in the yield ratio, is provided with excellent performance of shock prevention, and is stable in production process and strong in operability.