Identifying the source of pollution in groundwater and understanding the hydrochemical characteristics of contaminated groundwater by such pollution are very important for pollution prevention of groundwater. As the waste water of petroleum and natural gas industry, the produced water has the characteristics of complex components and great harmfulness. In view of the problems that there was less research on the contaminated groundwater by produced water, and the characteristics of the contaminated groundwater and the method to identify the pollution source were still unclear, the paper toke a polluted groundwater site in Yan’an as the research area, applied the methods of hydrogeochemistry and stable hydrogen−oxygen isotopes to describe the hydrochemical and isotopic characteristics of the groundwater contaminated by produced water, and compared the sodium−chloride coefficients, magnesium−chloride coefficients, desulfurization coefficients and carbonate balance coefficients of groundwater and reservoir water to identify the produced water. The results showed that the contaminated groundwater by produced water in this area was characterized by high TDS and depleted stable hydrogen−oxygen isotope; The chemical types were mainly Cl−Na type and Cl−Mg·Ca·Na type, and with the decrease of the influence of produced water, the groundwater changes from Cl−Na type to Cl−Mg·Ca·Na type and then to HCO3
−Na·Ca·Mg type. The relationship of ion proportions was more chaotic than that of normal groundwater and had no linear law. The sodium−chloride coefficients, magnesium−chloride coefficients, desulfurization coefficients and carbonate balance coefficients of polluted groundwater by produced water were all within the range of Chang 6 reservoir water, indicating that the relevant parameters for judging the conditions of oil and gas accumulation could be used to identify produced water. This study described the hydrochemical and stable hydrogen−oxygen isotopic characteristics of contaminated groundwater by produced water, and proposed a method to identify produced water by comparing the relevant parameters of groundwater and reservoir water, which were of great significance to the identification, recognition, investigation, monitoring and repair of polluted sites by produced water.