The method for calculating irrigation quota of alfalfa by FAO Penman-Monteith equation and timed-averaged crop coefficients was studied in detail. With the meteorological and the alfalfa experiment data provided by Beijing Yongledian Water-saving Center, the irrigation quota of alfalfa was calculated and verified. The irrigation quota in different hydrologic years of alfalfa under different irrigation techniques was estimated, which provided the reference basis for irrigation management and planning of alfalfa. The method could also be used to estimate other crop irrigation quota.