Abstract: The thermal deformation behavior of 00Cr11Ni8Co6Mo3 maraging steel under the deformation temperature of 850-1100 ℃ and strain rate of 0.01-10 s^-1 was studied by DIL805 phase change meter. The thermal working diagram and constitutive equation of the alloy were established, and the microstructure evolution law was studied. The results show that dynamic recrystallization occurs at deformation temperature ranging from 900 ℃ to 1100 ℃ and deformation rate ranging from 0.01 s^-1 to 0.1 s^-1. With the increase of deformation temperature and the decrease of strain rate, the flow stress and peak stress of the experimental steel are reduced, the dynamic recrystallization is more adequate, the dislocation density is reduced, the microstructure uniformity is improved, and the grain refinement is remarkable. The stable processing area of the experimental steel is deformation temperature of 1000-1100 ℃ and deformation rate of 0.01-0.14 s^-1.