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The effect of two-step sub-solvus heat treatment process on the γ "phase microstructure of a novel nickel-based powder metallurgy superalloy FGH4113A (WZ-A3) was studied. The cooling rate of large-size forged turbine disc is lower after forging, and the size distribution of γ" phases is from about 100nm to 4500nm. Small samples were taken from the disc for heat treatment experiments. When the samples were heated to 1000 ℃ and 1050 ℃, the total proportion of γ "phases decreased, and the evolution mechanism showed signs of Oswald coarsening mechanism and PAM mechanism. When the sample was heated to 1100 ℃, the intracrystalline γ" phase was completely dissolved. If two-step sub-solid solution heat treatment process was used, the sample was first heated to 1120 ℃ for 2h followed by rapid cooling, and then heated to 1000 ℃, 1050 ℃ or 1100 ℃, the total proportion of γ "phases decreased. The evolution of intracrystalline γ" phases was dominated by the Oswald coarsening mechanism, and its morphology and size were relatively stable during the heating process. The testing rods were cut from the disc and went through as forged state + 1000 ℃ (without heat preservation) + aging heat treatment and as forged state + 1120 ℃ (2h) + aging heat treatment respectively, and followed by tensile test at 550 ℃. The yield and tensile strengths of the latter one are significantly higher than the first one, and this can be treated as a reference for dual-performance heat treatment process design of large-size turbine discs.