Abstarct: In arid and semi-arid regions, due to low rainfall and overexploitation of resources, and consequent saltwater intrusion, the available fresh sources are reduced. Hydrological study and water budget are the first steps in identifying and assessing the region's water potential to provide management plans for the preservation of the fresh water aquifer. The altering hydrological regime, the extra drawdown and eventually the death of aquifers, are among the destructive effects of overexploitation of underground resources. Geysoor Plain is situated in the southern arid and warm parts of Khorasan Razavi province. The overexploitation of groundwater in this critical plain has caused a lot of challenges in terms of water quantity and quality. To study the hydrogeological properties and its effect on groundwater quality, the water level data from 18 piezometers and also water samples from some pumping wells in the plain was analyzed to measure the isotopic (18O, D, 37Cl and 81Br)، and hydrochemical characteristics. baxsed on the iso-potential map of the plain, the aquifer is being recharged from south, southwest, and northwest and discharged toward the northeast. According to the 23-year unit hydrograph of the plain, the average water level was declining, indicating a negative balance and a drop of 5.5 meters during this period. baxsed on the annual water budget, a deficit of 4.12 Mm3 was estimated for the plain. Geysoor plain was divided into three zones baxsed on 15-years of drawdown data. The lowest and highest drawdown is observed in the north and west and in the eastern part of the plain, respectively, due to the changing groundwater quality and number of pumping well. According to the EC map, the electrical conductivity fluctuations are almost consistent with the direction of groundwater flow, so that in the southern and northwest parts of the area, the lowest EC value is measured at 989 µS/cm. The highest EC was observed in the central areas of the plain, with a value of about 16800 µS/cm, indicating an intrusion of salt water from salt plain. Therefore, the overexploitation of the wells and consequent change in hydrologic regime is the main etiologic factor of the groundwater quality degradation in the central parts of the plain. Along the direction of groundwater flow towards the outlet of the plain, the salinity has been reduced due to recharging from carbonate units in the eastern regions.