It is difficult to determine straightening stroke precisely for automatic straightening machines. The existing calculation method of straightening stroke is based on straightening curvature equation. Because the initial curvature is obtained by curve fitting method, the method is inconvenient to use and its accuracy is not well. In this paper, a method to calculate straightening stroke based on straightening process model is put forward. So the mathematic load-deflection model of straightening process must be established at first. The straightening process model for shafts is developed according to the elastic-plastic mechanics theory. By using the model, the straightening stroke can be calculated directly according to the initial deflection of the bent part. It is advantageous to use and to decrease errors. Comparing with FEM results and experimental results, the calculation results by the model are exact and the errors are small. So the validity and the accuracy of the method are verified. It will be helpful to the development of automatic straightening machines.