Abstract: A kind of 550 MPa steel used for engineering machinery with low cost was prepared, and the effects of ultra fast cooling process after rolling on microstructure and mechanical property of the steel were studied. The results show that the ultra fast cooling process could increase the properties of the steel effectively, and the yield strength, tensile strength, elongation and impact energy at -20 ℃ were 600 MPa, 755 MPa, 19% and 253 J, respectively. The microstructure of the steel after ultra fast cooling was bainite, and the reinforcement phases were fine bainite and fine precipitation distributing in matrix.