Although analytical method is a practical method in the calculation of phreatic line of groundwater in reservoir bank slope, it is based on certain assumptions and the linearization of the Boussinesq's equation. Different mathematical models for the seepage of groundwater were established and then solved by analytical method or finite element method. By comparing the solutions to these different models, the errors caused by these assumptions and the linearization were analyzed. Results showed that both the errors caused by the linearization and the neglect of unsaturated seepage were greater than the error caused by assuming the  reservoir bank is vertical, and all of them decreased with the increase of permeability coefficient. The error caused by the assumption of uniform water level changes could be ignored in the case of Three Gorges reservoir water scheduling. When calculating the groundwater phreatic line in reservoir bank slope, analytical method only applies to the case when the water level fluctuation amplitude is smaller than 30% of the aquifer thickness, the geotechnical structure is regular, and the soil has good permeability and simple structure.