Based on analyses of SAGD field production data and numerical simulations,SAGD production process in thin heavy oil reservoirs was divided into periods: steam chamber horizontal expansion period and steam chamber downwards migration period. A model was built,which includes mass conservation equation,energy conservation equation and heat loss equation. A comprehensive mathematical expression was obtained to describe steam chamber development process. The expression is a typical Volterra integral function of the second kind,which could be solved by Laplace transformation. Comparisons were made between the model and CMG Stars'. A small relative error,less than 5% for the whole SAGD production process,was found. The model provides an accurate and quick method for determination of limited reservoir properties and proper production parameters for SAGD production.