Abstract: Finite element method is applied more and more widespread in modeling and management of groundwater with computer development. The space and the time must be treated to be discrete, although which are continuous. So the error occurs, which is not negligible sometimes. In this paper, on the basis of dealing with superposition principle and eigenvalue finite element method, the error due to different spatial and temporal discretization is seperated and analysed through a typical example, and an approach is researched to reduce the error.