The invention discloses a method for refining grains of large medium high alloy steel forgings in the technical field of metal heat treatment. The method comprises the following steps of: austenitizing the forged forgings, cooling to tip temperature of a perlitic transformation area, and isothermally keeping temperature or fluctuantly keeping temperature to realizing isothermal decomposition of pearlite; and cooling to room temperature, and performing the secondary austenitizing process to realize recrystallization to refine the grains again. The method solves the problem that the traditionalmultiple normalizing process in actual production has instable effect of refining the grains of the large forgings, and a serious phenomena of coarse grains and mixed grains. By incomplete isothermalbalanced decomposition of austenite, structure inheritance is cut off and eliminated, average austenite grain size above ASTM No.5 grade is achieved, the structure state of the large forgings is improved, the ultrasonic inspectability of the large forgings is improved; meanwhile, working hours are greatly shortened, the energy consumption is reduced, and the cost is saved.