According to the development status of computer aided tolerance design, a strategy and digital method for large-scale tolerance design of machine tool was proposed. In implementation, the error sensitivity analysis was confirmed as important basis of tolerance design, and the digital forecast of tolerance control effect was regarded as the direct guidance about tolerance inspection and adjustment. The theory and method of a general error model of machine tool was introduced. Making the plane as an example, the modeling method and expressing form of mathematical model of tolerance were supplied. A demonstration about forecasting method of tolerance control effect was given finally.