Abstract: The paper analyses the water budget(precipitation and evaporation)of the arid plateau in north of the Weihe River in Shaanxi Proyince.The precipitation data are collected from each station in this region for 49 years, and from Xian station from 1932 to 1980.The precipitation for different guarantee is obtained.The evaporation from water surface and the potential evaporation of the region are calculated by H.L.Penman’s formala. The annual actural evaporation of the region is calculated by H.P.Bailey’s formala.The concept of optimal evaporation is introduced and the surplusdefieit index of water is disueussed.The potential evaporation ealculatied by Penman’s formala being emended,the surplus-deficit amount of water is obtained. The principal conclusions are as follows.1)The annual precipitation is 525.97-641.1mm.It is increascent from east to west.80% of ensuring rate of precipitation is 470-568mm.2)The annual evapotation from water Surface and the potential evaporation are 920.1-1303.9mm and 748.0-1101.9 mm,respectively.They are decrescent from east to west.The maximum of annual variation occurs in June,and the minimum in December.The difference of annual actual evaporation is not large,471.6-516.8mm.The annual evaporation ratio is 0.79-0.92.It is also decrescent from east to west.3) The annual deficit-surplus amount of water is-215.1-15.0 mm.Water is surplus in autumn and deficit in the other seasons.