Abstract: Temperature field at different depth positions of the large size (200 mm×800 mm×2 000 mm) SR19 hot-working die steel during the annealing cooling process was simulated by using DEFORM software. According to the cooling curve, the spheroidization annealing process of the small size steel specimens was designed, that was, heating the sample to 860 ℃ and holding for 2 h, slow cooling at different rates (15, 30, 45, 60 ℃ · h⁻¹) to 740 ℃, respectively, holding at 740 ℃ for 4 h, then cooling to 500 ℃ in furnace and then air cooling to room temperature. The effect of cooling rate on the microstructure and hardness of specimens was analyzed. The results show that with the increase of cooling rate, the content of proeutectoid ferrite and the uniformity of carbide distribution in the specimen both decreased first and then increased, the spheroidization rate first remained stable and then rapidly declined, and the annealed hardness first decreased and then increased. When the cooling rate was 45 ℃ · h⁻¹, the microstructure was the best; the distribution of carbides was uniform, the spheroidization rate was relatively high, the content of proeutectoid ferrite was relatively low and there was a fallback in the hardness. The reason for the precipitation of proeutectoid ferrite was mainly due to the improper control of the cooling rate during annealing, which led to the preferential transformation of austenite into ferrite and retention of ferrite in the final microstructure.