In order to study and analyze the transformation structure of Nb microalloyed HRB400E high strength rebar at different cooling rates, and achieve the purpose of fine production management in the field, the CCT curve was drawn with the help of LINSEIS L78 thermal expansion instrument and AutoCCT software combined with the microstructure and hardness measurement. The results showed that F+P transformation occured at the cooling rate of the undercooled austenite less than 3.5 ℃/s, B was precipitated at the cooling rate of 3.5 ℃/s, no P was precipitated at the cooling rate of about 20 ℃/s, and the micro structure was B with a small amount of F. M phase transition occured at the cooling rate of about 32 ℃/s, and no F precipitates at this time, and all of them change to M at the cooling rate of about 80 ℃/s. With the increase of the cooling rate, the F grains became smaller, while the hardness increased and tended to be flat at the cooling rate of about 80 ℃/s. According to the measured CCT curve, the field rolling process was that the open rolling temperature was (1 040±30) ℃, the upper cold bed temperature was (950±30) ℃, and (990±30) ℃ for the size of less than 25 mm and more than 25 mm, respectively, the cooling rate after rolling was 2.23.0 ℃/s. And, high strength rebar with FP structure and good performance can be obtained.