Abstract: Taking high strength steel Q800 thick plates with thickness of 55 mm as research objects, the microstructure, properties and precipitation morphology of the tested steel in hot-rolled state and quenched and tempered state were analyzed. The results show that there were large amounts of lath bainites and granular bainites at 1/4 thickness of hot-rolled steel, the microstructure at 1/2 thickness were granular bainite and polygonal ferrite, and a large number of M/A islands existed in matrix. After quenching and tempering, M/A islands decreased sharply, lathes on the edge were thicker and the microstructure in the center were massive ferrite and granular bainite. Both tensile strength and yield strength of quenched and tempered steels dropped, but the elongation increased notably, and there was little change in hardness at cross section. Precipitations in the tested steel mainly were carbonitrides containing Nb and Ti, and the amount of precipitations in hot-rolled steel was few but increased in quenched and tempered steel and parts of them had a trend of growing up.