In this paper, the phase transition points of test steel and continuous cooling transformation (CCT) curve of supercooled austenite were determined with the FORMASTOR-F full automatic phase transformation instrument as well as the microstructure of specimen processed at different cooling rates was analyzed with the Zeiss microscope by taking the 3Cr corrosion resistant oil well pipe steel as the object of study. The experimental results showed that the phase transition points of 3Cr corrosion resistant oil well pipe steel A_c1 was 730 ℃ and A_c3 was 830 ℃. When the cooling rate was at 0.1~0.5 ℃/s, the microstructure at room temperature was proeutectoid ferrite + bainite, when the cooling rate was at 0.5~0.8 ℃/s, the microstructure at room temperature was ferrite + bainite + martensite, when the cooling rate was at 1~3 ℃/s, the microstructure was mixed martensite + bainite and the martensite structure was obtained when the cooling rate was greater than 3 ℃/s.