Formation water is one of the significant factors affecting the development of gas fields. The presence of water in gas wells can decrease the gas phase permeability of the reservoir, leading to a rapid decline in gas production and severely restricting the improvement of well productivity. Furthermore, there is still some controversy regarding the sources of formation water in study area. In this study, through an investigation of the formation water system in 11 production wells in the Xinchang gas field, hydrochemical and strontium isotopic analyses were conducted to examine the major and trace elements, water-rock interactions, and strontium isotope characteristics. The results indicate that the formation water in the 2nd member of the Xujiahe Formation gas reservoirs exhibits characteristics of low total dissolved solids (ranging from 55 166.00 to 122 547.41 mg/L), high bromine (Br) and strontium (Sr) contents (ranging from 642.00 to 1 711.00 mg/L and 670.00 to 1 780.00 mg/L, respectively), and ⁸⁷Sr/⁸⁶Sr ratios (ranging from 0.715 27 to 0.721 77). These characteristics suggest that the source of strontium isotope in the formation water is different from that of marine carbonate rocks and is more consistent with sandstone weathering. The enrichment of calcium (Ca) in the formation water is primarily related to the dissolution of authigenic minerals in the sandstone reservoir. During deep water-rock interactions, strontium replaces calcium in a cation-exchange manner and acquires bromine from organic matter, leading to an enrichment of bromine. These findings provide a theoretical basis for predicting the water-bearing horizons of the reservoir in the future.