Palmer Moisture Anomaly Index is based on the water balance equation.Based on a hydrological budget month-to-month for one area,the CAFEC(Climatically Appropriate for Existing Condition) quantities for evapotraspiration,recharge,runoff,water loss and precipitation can be computed,and the water departure can be obtained.As a water anomaly index,the water departure is not able to reflect the extent of drought or wetness.For this reason,Palmer put forward K(the weight factor reflecting climate characteristic).Using this method,the monthly Palmer Moisture Anomaly Index from 1961 to 2000 for Xi’an are computed and dryness/wetness grades for different month are evaluated year by year.Comparative results indicate that the Palmer Moisture Anomaly Index is more suitable for the semi-arid area in the east of Northwest China.