Weibull model is an important forecasting model, and it can forecast production and recoverable reserves. The model has three undetermined constants, which belong to the nonlinear model. In the past, the linear iterative or nonlinear iterative method is needed to solve it, however, the two methods above are complex and have some uncertainties. Based on the basic equations of Weibull model, the dimensionless formula is established by theoretical derivation, type curves for manual or automatic matching by computer are set up, which have only one undetermined constant. According to the matched model constant, the production, cumulative production and recoverable reserves of oil and gas fields can be forecasted. The application of field example indicated that, the content and methods in the paper are correct and effective.