In order to clarify the hydrochemical genesis and determine the aquifer levels of the fourth series groundwater in the Jidong Plain, the fourth series groundwater in the Tangshan Plain area was selected as the research object. The Piper trilinear diagram, Gibbs diagram, and ion composition ratio methods were used to explore the groundwater recharge sources, hydrochemical characteristics, and genetic mechanisms in the study area. Based on the Bayes discriminant method, a model for identifying the aquifer levels of the groundwater in the study area was established. The results showed that the hydrochemical types of shallow groundwater gradually changed from HCO3-Ca·Mg type in the mountain foot area to Cl-Na type in the coastal area, while the deep groundwater changed from HCO3-Ca·Mg type to HCO3-Na type. The formation of the hydrochemical components in the entire freshwater zone of shallow and deep groundwater in the study area was mainly influenced by rock weathering, with mineral dissolution as the dominant process. However, that in the brackish water zone of shallow groundwater was mainly controlled by evaporation concentration, with evaporite dissolution as the dominant process; and that in the brackish water zone of deep groundwater was mainly controlled by cation exchange adsorption. The aquifer level identification model based on the Bayes discriminant method can quickly and accurately identify the aquifer levels of the groundwater. This model can provide an effective approach for determining the pumping levels of groundwater monitoring wells and serve as a technical support for the formulation of aquifer level classification standards.