Abstract：By measuring phase expansion curves under different cooling rates, continuous cooling transformation curve (CCT curve) of 4Cr5Mo2V steel and Ac1, Ac3, Ms point temperatures were obtained. Combined with the phase change point and the results of ThermalCalc analysis, the insitu observation was used to study the microstructure changes of 4Cr5Mo2V steel during the heating, heat preservation and cooling process and change law of carbide during heating. The results show that the phase transition temperature of 4Cr5Mo2V steel is 820℃ for Ac1, 855℃ for Ac3, and 275℃ for Ms. During heating process, Cr23C6 dissolves before VC and produces emboss and holes on the surface of steel. During insulation process, grain growth mechanism is migration of primary grain boundary and the grains are swallowed and grown. During the continuous cooling process, the martensite shape of the 4Cr5Mo2V steel is formed by gradually forming parallel slats on the basis of the first formed slats to form a slat bundle, or the slats formed first can trigger the other direction (60° or 120°) lath martensite, which constitutes a geometric feature such as a polygon.