It is not easy to carry out the detailed variable condition calculation of steam turbine in engineering application. In this paper, a variable condition calculation model based on the internal and external characteristics of steam turbine is proposed, and a variable condition calculation model of constant power and constant flow is established. The model is applied to calculate 75% THA, 50%THA, typical industrial and heating extraction conditions of a subcritical 330 MW unit. The error is small compared with the design value, and the calculation accuracy meets the requirements. The results show that the model has high accuracy and can meet the requirements of engineering application.