Blade which transfers thermal energy of steam into power, is a basic component in steam turbine. The reliability of blade is heavily influenced by the operating environment. The rotating blade experiences large inertial load and the wake of nozzle flow impose large variations of aerodynamic load on blade, in addition, the last stage blade is also affected by corrosion, so accidents of blade happen from time to time. Preventing blade failure has become one of the major objectives of turbine design and in-service maintenance. It is said that the reason for most of blade failure is fatigue fracture. In this study, a synthetical numerical model has been developed to evaluate service life of blade. At first, a numerical model to analyze the excitation force, dynamic frequency and dynamic stress of steam turbine blade has been developed, based on the results of dynamic stress analysis, a model to evaluate the service life of turbine blade has been developed. Many factors such as manufacturing technology of blade and erosion operating environment are considered to get more accurate results for service life of blade. At last, a last stage blade group of a large power steam turbine is analyzed in detail. It is shown clearly that the numerical model can give some rational quantitative results, and it is suitable for its engineering application to the improvement of the blade reliability.