<p>Based on the Ginzburg–Landau theory and crystal plasticity theory, considering the influence of grain size and temperature on the ratchetting of medium-manganese Transformation-Induced Plasticity (TRIP) steel, a two-dimensional elastoplastic phase-field model was established by incorporating grain size and temperature-dependent dislocation slip resistance and phase transformation energy barrier. Subsequently, the ratchetting behavior of medium-manganese steel under asymmetric stress-controlled loading was simulated using the proposed model, and the microstructure evolution and the interaction between phase transformation and plasticity were revealed. The simulated results show that both the ratchetting strain and martensite content decrease with decreasing grain size or increasing temperature. The smaller the grain size, the more stable the austenite, and the more difficult the martensitic transformation. In addition, increasing temperature also impedes the martensitic transformation. Due to the inhibition of martensitic transformation, the local stress concentration is weakened and the deformation is more uniform in the polycrystalline system of medium-manganese steel. Therefore, decreasing grain size or increasing temperature will inhibits the TRIP effect and decelerates the evolution of the ratchetting strain of the medium-manganese steel.</p>