The commonly used IA (intercritical annealing) and QP (quenching and partitioning) processes for medium manganese steel result in microstructures of austenite + martensite and austenite + ferrite. The former presents challenges in enhancing plasticity, whereas the latter struggles to improve yield strength, leading to a bottleneck in mechanical properties. To develop a new generation of medium manganese steel, it is essential to introduce new phase structures, among which bainitic transformation has garnered attention because of its favourable combination of strength and plasticity. However, the relatively high manganese content in medium manganese steel results in a relatively slow rate of bainitic transformation. Therefore, a method involving cold rolling of undercooled austenite prior to isothermal treatment was employed to introduce dislocations and accelerate the occurrence of bainitic transformation. Research findings indicate that after predeformation treatment, the bainitic microstructure appears 24 h earlier in deformed samples than in undeformed samples, with the transformation being nearly complete at 60 h, which is 12 h earlier than that in undeformed samples. In this experiment, dislocations do not hinder bainite growth; rather, the reduction in final bainite content is attributed to the significant promotion of pearlite nucleation and growth by dislocations. The extensive formation of pearlite consumes the nucleation sites at the grain boundaries, resulting in a decrease in the final transformation fraction of bainite from 40 vol.% to 18 vol.%.