The efficient monetary system requires that the money supply is flexible enough,in order to face the change of the money demand.If the money supply can not be changed while there is a sustained increase in the money demand,it will produce a shortage of money.This paper,based on a simple cash-in-advance model,studies the monetary system of the Song Dynasty,and expects to analyze the intrinsic reason of the money shortage prevailing in the Song Dynasty.Chinese ancient copper coins system does have its particu...