Abstract: With the public choice theory, Buchanan studies on the contractual rules and establishes the functional relation between the public base and feasibility of government, as has expanded the applying scope of economics and also enhanced the interpretive nature of political science. However, Buchanan' s democratic theory is not yet perfect, for the concept of taking the cost as the standard makes it fall into a difficult position in interpreting the majority rules common for constitution. The concept of establishing the new governmental decision-making model and the replacement of Buchanan' s theory with net profit standard can make up the defects of Buchanan's theory in interpretation upon reality.