Abstract: In recent years, the concentration of sulfate in groundwater in northern karst areas has increased significantly, which not only poses a potential threat to human health, but also affects the safety of industrial and agricultural water and the sustainability of ecological environment. However, the pollution mechanism is still unclear. This paper studied the hydrogeochemical process of groundwater in Tai’an City, Shandong Province. In order to reveal the hydrochemical characteristics of surface water and hydrochemical process of groundwater, the mathematical statistics, hydrochemical analysis, hydrogen and oxygen isotopes (δD, δ¹⁸O-H₂O) and sulfur and oxygen isotopes (δ³⁴S-SO₄, δ¹⁸O-SO₄) analysis are comprehensively used. Moreover, the relationship between the increase of sulfate concentration and the hydrochemical component were discussed and the isotope characteristics and sources of sulfates were clarified. The results show that the hydrochemical type of groundwater is mainly HCO₃·SO₄·Cl-Ca and SO₄·Cl-Na·Ca type, showing the hydrochemical components are mainly affected by the dissolution of carbonate minerals and human activities. Groundwater and surface water mainly come from atmospheric precipitation, which is affected by evaporation. The influence of evaporation intensity on surface water is higher than that on groundwater. The δ³⁴S-SO₄ and δ¹⁸O-SO₄ in groundwater are between 4.5‰-8.5‰ and 2.6‰-6.6‰, respectively, which is close to the end element of sewage discharge. The analysis of the relationship between δ³⁴S-SO₄ and \mathrmSO_4^2- content shows that the reduction of sulfate by bacteria in groundwater is weak, and the effect of sulfide oxidation on \mathrmSO_4^2- in groundwater is also weak. \mathrmSO_4^2- is mainly derived from sewage discharge of industrial enterprises.