According to the monitoring data of inflowing and outflowing of Lake Erhai Basin in 1998,2004-2006,through measured flow,precipitation and evaporation,we calculate and analyze the error sources of inflowing and outflowing water,and propose the problem required to solve for the increasing calculation accuracy of water amount of Lake Erhai,thus provide a more accurate and scientific technology support for water environment, water resources protection and predictive analysis on the amount of inflowing and outflowing water of Lake Erhai.