The Kelasu structure belt in Kuqa depression, Tarim basin has complex structural characteristics and lacks systematic research on chemical characteristics of formation water and fluid system. In order to clarify the chemical characteristics and fluid system of formation water in Kelasu structure belt, the characteristics of salinity, ion proportion coefficient and pressure distribution of formation water are analyzed. The results show that the formation water of Kelasu structural belt is mainly brines with high salinity, which are classified as CaCl2 type according to Surin classification, Cl- Na type and Cl- Na·Ca type according to Schukarev classification, and are typical sedimentary buried water. The formation water has low desulfurization coefficient and high metamorphism coefficient, indicating that the formation water has experienced strong water- rock reaction. According to the characteristics of formation pressure field, lithology and formation water, it can be divided into four fluid systems in the study area: Quaternary- Neogene, Paleogene, Cretaceous, Jurassic and its underlying strata. Based on the analysis of lithologic composition, structural evolution characteristics and chemical characteristics of formation water in the study area, it is concluded that the chemical characteristics of formation water is influenced by the mixture of organic transformation fluid, sedimentary water and high salinity fluid of Paleogene and hydrocarbon fluids from Jurassic source rocks during hydrocarbon generation.