Aiming at the outstanding problems of flexible operation in full working conditions of contemporary large steam turbines, a new dynamic stress simulation analysis method for the long blades was proposed by taking the low-pressure last stage blade of a 1000 MW air-cooled steam turbine in Harbin Turbine Company Ltd. as the research object. The validity of the calculation method was demonstrated by the long blade dynamic test. The theoretical analysis shows that the vortex-induced effect on the blade surface caused by flow separation is a key factor to increase the dynamic stress of the blade under low load operating conditions. For the engineering application, it is feasible and effective to calculate and analyze the dynamic stress of the blade under different working conditions by taking the pressure fluctuation value on the blade surface of the aerodynamic flow field as the excitation factor for the structural field excitation force. By comparing several schemes, the simulation results are in good agreement with the test results. Both of the simulation and dynamic test results show that the maximum dynamic stress of the last stage blade appears at about 20% load, and the peak value of the dynamic stress of the blade is about 5% of the allowable value, which has sufficient vibration resistance strength and safety margin, and can well meet the requirements of flexible, safe and reliable operation of the large steam turbine under full working conditions.