Abstract: The industrial experiment of φ70 mm 45 steel by water-piercing cooling after rolling was carried out in self-made water-piercing equipment, and the effect of water-piercing cooling after rolling on mechanical properties and microstructure were comparatively studied. The results show that the tensile strength and yield strength of the steel after water-piercing are increased to a certain extent, in which the tensile strength is increased from 630 MPa to 659 MPa, the yield strength is increased from 345 MPa to 365 MPa, but the elongation, reduction of area and hot rolling hardness have no significant increase. When the temperature of the steel surface drops to 730-680℃ after water-piercing, Widmanite and large pearlite begin to appear in local areas of the steel surface; when the surface temperature of the steel drops to 680-553℃ after water-piercing, granular Bainite and massive Pearlite appear on the surface of the steel. In addition to significant difference in the surface microstructure, strong and weak cooling has a small effect on the microstructure of 1/2 radius and the core area of the steel.