The invention relates to a thermal treatment process for improving microscopic coarse grains and microstructures of an H13 forged piece for an H13 aluminum extrusion mold with high requirements for microscopic and post-forge annealing structures. The process comprises high-temperature diffusion heating, high-temperature diffusion cooling, normalization heating, normalization cooling and isothermal spheroidizing, wherein two times of preheating is adopted in the high-temperature diffusion heating, and fog cooling is adopted in the normalization cooling. According to the process adopting a high-temperature diffusion, normalization and isothermal spheroidizing mode, local coarse grains are eliminated by high-temperature diffusion, so as to obtain relatively large and uniform grains and structures ready for subsequent normalization and isothermal spheroidizing; fog cooling is adopted in the normalization cooling to increase the cooling speed, so that the coarse grains produced by high-temperature diffusion are thinned, and netlike carbide structures are eliminated; and through the isothermal spheroidizing, fine and uniform globular pearlite and a small amount of diffused granular carbides are obtained, and the purpose of eliminating the microscopic coarse grains and the microstructures is achieved.