Abstract: The effects of solution and aging on the microstructure transformation and mechanical properties of S51740 stainless steel were studied. The results show that the microstructure of S51740 stainless steel after solution and aging treatment is mainly tempered martensite, δ-ferrite and the second phase precipitated from martensite matrix. When the aging temperature is 480 ℃, the δ-ferrite increases gradually with the increase of solution temperature, and its strength and toughness increase gradually, and they reach the highest at 1070 ℃. When the solution temperature continues to increase, its strength and toughness decrease, and the toughness decreases obviously. The sample is aged after solution treatment at 1070 ℃. With the aging temperature increasing, the δ-ferrite does not change, while the second phase gradually increases and accumulates, its strength gradually decreases, and the toughness gradually increases. At 640 ℃, the strength continues to decrease while the toughness continues to increase.