A method for computing the water departure based on the CAFEC(Climatically Appropriate for Existing Condition) precipitation is introduced.Based on a hydrologic budget month to month for one area,the CAFEC quantities for evapotranspiration,recharge,runoff,loss water,and precipitation can be computed, and the water departured can be got.As a index of water abnormity ,the water departure d is more fit for measuring water profit and loss for the same period by years.Using this method,the monthly water departures from 1961 to 2000 of Beijing are computed,and the changes of water departures of January,April,July,and October year by year are analysed.