The C content in high strength steel must be controlled at a lower level for the good weldability. However, the lower level of C content will reduce the C partitioning efficiency and influence the stability of retained austenite, which leads to the decrease of the product of tensile strength and elongation of high strength steel. A novel preparation mechanism of high strength steel is to employ some kind of substitutional alloying elements, for example Mn, instead of C to partitioning to enhance the austenitic stability, which would not remarkably reduce the weldability of the steel. One low alloy C-Si-Mn steel was used in present work. The Mn partitioning behavior and its effect on the stability of the retained austenite and the mechanical property were studied by means of intercritical annealing, subsequent austenitizing, then quenching and partitioning process (I&Q&P). The results show that in the process of intercritical annealing at 760 ℃, by extending the annealing time, austenite volume fraction increases gradually until it reaches the saturation, meanwhile the Mn partitioning behavior occurs and Mn content increases gradually from ferrite to austenite until it reaches the chemical potential balance in two phases. The sample is heated to 930 ℃ for 120 s, then rapidly quenching to 220 ℃, the carbon diffuses from martensite to austenite phase in the process of partitioning. After I&Q&P process, the tensile strength of experimental steel is 1310 MPa, elongation up to 12%, the product of strength and elongation up to more than 15000 MPa.%. The steel only contains a small amount of retained austenite by only C partitioning after traditional Q&P process, while the steel contains more Mn-rich retained austenite after I&Q&P process. Hence, the content and stability of retained austenite of steel can be improved significantly, which enhance the formability at room temperature.