Abstract: 150 mm thick S460G1 steel heavy plate was obtained by hot rolling after strictly controlling rolling process, and then quenched at 870—960 ℃ and tempered at 550—660 ℃. The microstructure evolution and strengthing-toughening mechanism of the steel plate were researched. The results show that from the surface to the center of hot rolled steel plate, the microstructure was acicular ferrite→granular bainite→ferrite and pearlite. After quenching, the microstructure on the plate surface was lath martensite, one quarter thickness of the plate was mainly granular bainite and the center of the plate were ferrite and pearlite. After quenching at 900 ℃ and 930 ℃, the original austenite grains at the one quarter thickness of the plate were fine and uniform, and the grain size distribution was concentrated with ranging of 4.12—31.88 μm and 5.02—32.69 μm, respectively. After quenching at 930 ℃ and tempering at 600—630 ℃, the microstructure on the plate surface was tempered sorbite, and the microstructures of one quarter thickness and center of plate were similar to those at quenching state, but the trend of equiaxed lath ferrite and coarsened ferrite was strengthened; at this time the steel plate had excellent comprehensive mechanical properties with the yield ratio of not higher than 0.85. Martensite/austenite(M/A) island decomposing, a large amount of precipitate, dislocation eliminating and equiaxial ferrite deforming were the main reasons for the excellent strength and toughness of the hot rolled steel.