The invention relates to a slow cooling method for a blank, in particular to a slow cooling method for a Ni and Cu alloy containing large extrusion thick-wall blank. The slow cooling method comprises the steps that firstly, blank manufacturing is conducted; and secondly, the slow cooling step is conducted and includes blank furnace feeding, material staying, heat preservation, furnace cooling and furnace discharging. The blank is manufactured under a 150 MN blank machine through three-phase inner stress, at the moment, the inner stress of the blank is large, white points are likely to occur, the surface hardness is high, and machining is hard; the inner stress of the blank is eliminated through the slow cooling step, the surface hardness is reduced, the inner structure of the blank is optimized, the hydrogen diffusion effect is achieved, and generation of stress cracking and white point structure defects is avoided.