Abstract: The astern steam braking capacity of marine steam power plants is a fundamental guarantee for the emergency braking ability of steam-powered ships. By analyzing the operating characteristics of the high-pressure and low-pressure rotors under variable working conditions during the astern steam braking process, torsional stress models of the rear journals of the high-pressure and low-pressure rotors are established. In addition, a Simulink-based simulation model of the torsional stress at the rear journals of the rotors is developed. The simulation results indicate that during the astern steam braking process of the steam power plant, when a full steam admission strategy involving simultaneous steam admission to the ahead and astern turbines is adopted, the torsional stresses at the rear journals of the low-pressure rotor exceed the allowable safety limits. This may result in a reduction in the service life of the rotors or even lead to fracture. The torsional stress analysis of the rotors during astern steam braking of marine steam power plants provides a basis for optimizing steam braking control strategies for steam-powered ships and offers a reference for improving ship maneuverability.