Abstract: For the creep failure of main steam pipe welded joints in supercritical thermal power plant, the experiments combining with numerical simulation were adopted for studying the formation and growth rules of creep voids and the creep mechanical behavior of 9Cr-1Mo heat-resistant steel welded joints. Specimens of parent material, welded joints and fine grained zone by thermal simulator were crept at 550 ℃, 600 ℃ and 650 ℃ under the loads of 40 MP-220 MPa. The creep specimens of thick plate welded joint were interrupted at 1010 h, 2000 h, 4425 h, 6000 h, 7020 h and 7970 h to observe and analyze the generation location as well as the cumulative amount of creep void, and it was compared with the distribution and evolution of mechanical parameters by finite element simulation. The results show that the creep strength of 9Cr-1Mo heat-resistant steel welded joints is lower than the base material, higher than the fine-grain zone under high load conditions.With the reduction of the load, the creep strength of welded joints is close to the fine grain zone gradually. Creep voids begin to form in the fine-grained zone and mixing-grained zone of welded joint in early stage of creep, with the increase of creep time, the voids continue to form, gradually grow up and gather in series, and finally form Ⅳ-type cracks. There is a certain negative correlation between the creep holes and the hardness in the heat affected zone of welded joints of 9Cr-1Mo. The high temperature creep environment leads to a substantial decrease in the hardness of the fine-grained area and the mixing-grained zone, and the effect of creep temperature on the hardness is more significant than creep time. The multi-axial stress state and the maximum principal stress are the dominant mechanical factor of the formation and growth of creep voids in 9Cr-1Mo heat-resistant steel welded joints.