Abstract: During solution treatment, two cooling methods, oil cooling and slow cooling, were used to simulate the surface and core microstructures of a large section new prehardened plastic-working die steel SDP1Cu. The effects of different aging treatments (350-600℃ for 1-20 h) on the microstructure and hardness were studied, and the aging precipitation strengthening mechanism were discussed by resistance method and aging theory. The results show that with increasing aging temperature and time, the oil-cooled martensite and slow-cooled bainite structure gradually decomposed and finally disappeared, and cementite particles were dispersed on ferrite matrix. After aging at 500℃ for 2-20 h, the hardness difference between oil-cooled and slow-cooled steels was within 0.5 HRC, and the cross-section hardness uniformity was good. The copper-rich precipitates started to precipitate when the temperature was higher than 400℃. With increasing aging temperature, the size of precipitates increased, and the precipitation strengthening mechanism changed from dislocation cutting mechanism combining modulus strengthening and coherence strengthening to dislocation bypassing mechanism.