Abstract: Temperature was selected as variable to study phase transformation in weld metal of 12Cr1MoV steel and realized in-situ detection of temperature in weld metal.The maximum temperature and cooling rates during welding were acquired to form the simulation technology of 12Cr1MoV steel in weld metal,and the SW-CCT diagram of 12Cr1MoV steel were obtained by the thermal simulation equipment with extracted cooling rates.When the average cooling rate of t_8/3 is under 1.7℃/s,the microstructure in weld metal of 12Cr1MoV steel is ferrite(F)+perlite(P)+bainite(B);under 4.7℃/s,the microstructure is P+B;under 33℃/s,the microstructure is B;over 33℃/s,the microstructure is martensite(M).The optical microstructures of typical cooling rates are coincident with the test results of SW-CCT.The Vikers hardness increases with the increasing of cooling rates.