The stress-strain curves of forged GH4742 superalloy at deformation temperature of 1020~1150 ℃ and the strain rate of 0.001~1 s-1 under the true strain of 0.65 were obtained by single-pass isothermal compression experiments, the hot deformation constitutive equation and hot working diagram of GH4742 superalloy were constructed, and the evolution of the micro-substructure and γ′ phase during the thermal deformation process were studied by SEM, EBSD, and eventually the correlation of the deformation process conditions-the microstructure difference-the mechanical property variety was established. The results show that the evolution mechanism of the alloy"s microstructure and mechanical properties were closely related to the Z parameter. When the strain rate increased from 0.001 to 1 s-1 at low temperature deformation of 1080 ℃, the lnZ value increased from 75.6 to 82.6, the thermal effect was strengthened, which resulted in the proportion increment of dynamic recrystallization, and the proportion of low-angle grain boundary decreased, the hardness of the matrix increased due to the refinement of the microstructure; With increasing strain rate at high temperature deformation of 1110 ℃, the lnZ value increased from 74 to 78.5, the dislocation slip and grain boundary migration were suppressed, which decreased the proportion of dynamic recrystallization, and the proportion of the high-angle grain boundary was improved, and the hardness of the matrix increased due to the work hardening. The critical lnZ value of GH4742 superalloy without grain coarsening of dynamic recrystallization was 73. The optimum hot working parameters for forged GH4742 superalloy was 1110～1150 ℃ and 0.001～0.01s-1 by the analysis of thermal processing map and the hot deformation microstructure.