Just as other economic variables, the scale of the foreign exchange reserves must be moderate. But how to measure the demand for foreign exchange reserves and keep the scalesmoderate is a difficult p roblem. Many economists at home and abroad focus research on this p roblem. In this paper, the author introduced the quantitative theories to analyze the issues of foreign exchange reserves, which include p roportion analysis, regression analysis and cost - benefit analysis. These theories offered some app roaches to analyze the app rop riate scale of the foreign exchange reserves.