Abstract: In order to solve insufficient hardness problem of 0.8mm thick cold-rolled AISI 631 semi-austenitic precipitation hardened stainless steel thin strip by the original heat treatment process, the effects of the prior solution treatment（1050℃ ×（1-7） min） on the microstructure and mechanical properties of the final heat-treated thin strip based on the original aging heat treatment process were investigated. The results show that there are considerable residual austenite γ and δ ferrite phases in the cold-rolled raw material due to inhomogeneous composition. Since austenite phase cannot be completely transformed into martensite via the original condition and aging heat treatment process, the hardness is lower. After 1050℃×（1-2） min solution heat treatment, however, the carbide and other alloy elements are fully dissolved to form uniform austenite, and then the austenite is almost completely transformed into fine martensite during conditioning together with high density of fine Ni Al strengthening phase precipitation after aging treatment, which results in the highest hardness and strength of 631 stainless steel.Nevertheless, if the solution time is further increased, the nuclear rate of martensite decreases and the hardness and tensile properties of 631 stainless steel decrease instead.