In this paper,optimal estimation model based on the theoretical framework of optimal size of government expenditure and Barro Model is built,and according to the time series data (1983-2007) in China,we find out that the appropriate size of government expenditure-GDP ratio is 34.66%.The optimal dynamic path of government expenditure is further described according to Dalton's principle of the greatest social benefits.Reference and analysis of ideas in this research are provided to research the changes in the optimal size of government expenditure.