Isothermal constant strain rate compression experiments were conducted on superalloy GH4742 by using simulator Gleeble-3500. The results show that γ′ phases of as-received materials exhibits three morphology sizes. When the strain rate is small (0.001 s-1), γ′I and γ′II phases are completely dissolved at the temperature greater than or equal to 1090 ℃, γ′III phase is completely dissolved at the temperature less than or equal to 1020 ℃. The equivalent diameter and volume fraction of γ′I phase increase with the increasing of temperature from 950 ℃ to 1020 ℃, but decrease when it rises to 1055 ℃. The equivalent diameter of γ′II phase decreases with the increasing of temperature from 950 ℃ to 1055 ℃, but the volume fraction does not change much. The equivalent diameter of the newly precipitated γ′III phase does not change much from 1055 ℃ to 1160 ℃. When the strain rate is large (1 s-1), the influence law is roughly the same as that at 0.001 s-1, except that the temperature at which γ′II phase is completely dissolved drops to 1055 ℃, and the volume fraction of the newly precipitated γ′III phase increases compared with that at 0.001 s-1. Thus, in order to fully dissolve γ′ phase to reduce deformation resistance, the initial forging temperature of superalloy GH4742 should be no lower than 1090 ℃.