As humanity continues to explore deep Earth, human activities are significantly changing deep-earth environments. Hydraulic fracturing has opened new avenues for studying water–rock interactions in deep reservoirs. In tight sandstone reservoirs enriched with formation water, what interactions occur between the fracturing fluid, tight sandstone, and formation water during hydraulic fracturing? Based on field fracturing test flowback fluid analysis results and laboratory experiments, this chapter characterizes and reveals the chemical processes of fracturing fluid-tight sandstone-formation water interactions in tight sandstone reservoirs during hydraulic fracturing. Focusing on the water-rich characteristics of tight sandstone reservoirs, a detailed study was first conducted on the interaction between fracturing fluid and formation water. Moreover, by simulating the mixing process of fracturing fluid and formation water, the interference of formation water mixing on water–rock interactions was eliminated, enabling the identification of processes such as mineral dissolution, mineral precipitation, redox reactions, and ion exchange during hydraulic fracturing.