Abstract: Abstract：The western Longdong basin is situated in an overthrust belt on the western edge of the Ordos basin, where Neoproterozoic-Lower Paleozoic carbonate rocks are exposed or buried at shallow depths, forming a N-S-trending karst water concentration zone. The karst-fissure water system is analyzed and discussed intensively by using the method of structural control on water and hydrochemical isotope analysis. The spatial distribution of karst water in the area is controlled obviously by a N-S-trending overthrust belt, the water-storing space is mainly karst fissures and the karst development and karst water concentration are mainly controlled by structure. The hydrochemical features of karst groundwater in the south are notably different from those in the north. The hydrochemistry in the Pingliang-Huating area in the south-central part with superior recharge conditions of groundwater is mainly determined by the aquifer lithology and its occurrence conditions. Karst groundwater mainly originates from meteoric water, while the buried karst groundwater is mixed water from multiple sources. Based on the spatial distribution, features of the hydrodynamic field and recharge, runoff and draining conditions of karst groundwater, the whole karst area is divided into three relatively independent karst groundwater systems, namely, the Pingliang, Huating and west Huanxian systems, which may serve as a basis for water resource evaluation in the karst area. These views have great scientific significance for a better understanding of the characteristics of occurrence and concentration and mechanism for formation and evolution of karst water in arid and semiarid areas of northwestern China and exploitation and utilization of karst water resources in the study area.