Abstract: An alternative simple model was proposed to compute dynamic stress in the blades of steam turbines.By the model and transient finite element method,dynamic stress in the last stage rotating blades of a air cooling steam turbine under various back-pressure was analyzed under centrifugal loads and steam pressure obtained by 3-D transient fluid analysis.Dynamic displacement response of dangerous points in the blade was also discussed by the harmonic response analysis.The results suggest that the 4th~7th resonant frequencies should be avoided inthe design of those kinds of blade.