To ensure reliable power supply of diesel generator set at all working conditions, especially some extreme conditions, shaft torsional vibration calculation is carried out for the natural frequency, the steady-state basic stress and the additive stress at instantaneous overload condition for a marine diesel generator set the shaft, based on steady-state and instantaneous torsional vibration analysis of the shaft. Comprehensive study on the operational security of diesel generator set shaft at instantaneous overload condition can assure the reliable all-condition power supply.