The invention discloses a slow cooling method for a large-scale extrusion thick-wall blank-making piece containing Cr and Mo alloy heat-resistant steel. The slow-cooling method comprises the steps that blanks are fed into a furnace, the blanks are waited, the heat preservation is carried out, furnace cooling is carried out and discharging air cooling is carried out, wherein the blanks are air cooled to 500 DEG C or above to be put into the furnace, the wall thickness of the blanks is 200mm-500 mm, the temperature of the waiting blanks is greater than or equal to 500 DEG C, the heat preservation temperature is 600 DEG C-850 DEG C, the heat preservation time is 25hours-55hours, and the furnace cooling speed is smaller than or equal to 40 DEG C / h. According to the slow cooling method, the problems of high surface hardness, stress cracking and white point tissue defects caused by slow cooling to room temperature from high-temperature heat state by low-alloy and high-alloy various blank-making blanks are solved, and the problem that the surfaces of the blanks are too hard or scrapped when the blanks are cooled to room temperature from high temperature is avoided.