In this study, we proposed an interrupted intercritical annealing (IIA) strategy to produce high-strength medium Mn steel with a nominal chemical composition of Fe–0.20C-8.0Mn–1.6Si–1.3Al (wt.%), which can promote the dissolution of carbides and make the C and Mn elements fully diffuse into austenite to enhance the stability of austenite. Furthermore, the content of retained austenite (RA) was significantly improved while avoiding excessive growth of RA by controlling the nucleation and growth of austenite. The concentration gradient of Mn was established to form the core-shell structured RA which had a higher Mn content in the shell than in the core. We used IIA process to prepare medium Mn steel with an ultimate tensile strength (UTS) of 1536.93 MPa, a total elongation (TE) of 43.26 %, and a product of strength and elongation (PSE) of 66.49 GPa·%, which exhibits excellent mechanical properties. This study provides a theoretical reference for the preparation of high-strength medium Mn steel.