The invention provides medium-manganese TRIP (Transformation-Induced Plasticity) steel and a preparation method thereof. According to the medium-manganese TRIP steel and the preparation method thereof, medium-manganese cold rolled steel is taken as a raw material; the cost is low; the performance of a final product is realized by adopting critical region heat preserving, quenching cooling and partitioning processes; the structure of the medium-manganese steel is controlled by utilizing an over-aging stage in a continuous annealing production line, and the residual austenite volume fraction of the structure is improved in a relatively short critical annealing time, so that mechanical properties of a product are ensured, and meanwhile, the cost is reduced as much as possible; the performance of the steel is stable, the process fluctuation has little influence on performance, and industrial production is facilitated; and the prepared medium-manganese TRIP steel is high in strength, high in elongation and good in forming property, can be widely applied to reinforcing structural parts, such as a B-pillar reinforcing plate, to improve the collision safety of an overall vehicle, and achieves the effects of reducing weight and reducing emission.