Based on thermal simulation, pilot test, metallographic observation, tensile test, TEM and thermodynamics software, effects of cooling rate and rolling process on microstructure, mechanical properties and precipitation of Nb, V microalloyed 600MPa grade high strength steel rebar HRB600 were studied. The results show that the microstructure of the tested steels are composed of ferrite and pearlite when cooling rate is lower than 3℃/s; bainite is grown when cooling rate is 5℃/s, and with the increase of Nb content, the bainite content increases. The air cooling and light water cooling after finishing rolling, the microstructures of the tested steels are ferrite and pearlite, and the pearlite content(by mass) is about 42%47%. With the increase of Nb content and cooling rate, as well as the decrease of rolling temperature, ferrite grains and pearlite lamellar spacing decrease. Nb is precipitated with square, circular and bar shape, and with the increase of Nb content, the size of the precipitates increases. The content of Nb should be controlled within 0.04%(by mass). And accelerated cooling after finishing rolling to 8001000℃ should be used to prevent the coarsening of Nb precipitates, and cooling rate should be controlled 13℃/s in the phase transition temperature range.