Daweijia wellhead field is located at the coastal karst area in north China. Unsustainable groundwater exploitation in recent decades has resulted in severe seawater intrusion in this area. Employing hydrogeological investigations, hydrodynamic monitoring and hydrogeochemical and isotope data analysis, this study analyzed the groundwater hydrodynamic conditions partially controlling seawater intrusion processes, and delineated the mechanism of seawater intrusion in this area. The characteristics of major hydrochemical composition and multi-stable-isotopes (δ²H-δ¹⁸O, δ³⁴S, δ¹³C) in groundwater have been used to identify the main hydrogeochemical behaviors; these were also quantitatively simulated by inverse hydrogeochemical reactions. Seawater intrusion occurs primarily along the permeable fault zone, karst fissures and loose Quaternary strata. The compositions of  δ¹⁸O and  δ²H in groundwater show that the critical groundwater recharge source is precipitation. Combined with the Cl- distribution, it can be identified that besides salinity increase accompanying seawater intrusion into fresh water aquifer, evaporation of shallow groundwater also plays an important role in the accumulation of salt in groundwater. Joint analysis of δ³⁴S_SO4, δ¹³C_HCO3composition with special anions (e.g., SO^2-₄，Cl^-) reveals that brackish and saline groundwater are not the results of a simple mixture between seawater and fresh groundwater. Inverse hydrogeochemical modeling can be used to reveal the main hydrogeochemical reactions controlling the hydrochemical evolution of coastal karst aquifers in the study area, including the dissolution of calcite, montmorillonite and gypsum, the precipitation of illite and Ca-Na cation exchange with CO₂ extrication.