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.