Abstract: DP600 steel with different Mn content was designed and industrially produced under different laminar cooling air cooling time. The effects of Mn mass fraction (0.65%-1.25%) and laminar cooling air cooling time (8-11 s) on microstructure and strength of the steel were studied. The results show that the average grain size decreased, and the martensite content, the yield strength, and the tensile strength increased with increasing Mn content or decreasing laminar cooling air cooling time. With the increase of Mn content, the morphology of martensite changed from dispersion to chain. When the mass fraction of Mn was 0.85%, and the laminar cooling air cooling time was 8-9 s, DP600 steel had the excellent tensile properties with yield strength of 441-462 MPa, tensile strength of 614-618 MPa, and the martensite area fraction in the microstructure was less than 20%.