In view of the 330MW subcritical condenser steam unit with single intermediate reheat, four cylinders and quadruple flow, the large-area cracking problem on the 4th-stage blade of LP rotor was analyzed. Main factors contributing to the vibration response and dynamic stress of blades were discussed. With the vibration model, the vibration features and dynamic stress were calculated. The result showed that the blade fracture was mainly caused by the defective design in blade vibration characteristics.