Borehole hydraulic head observation data is essential to the inversion of hydraulic seepage parameters. In order to avoid re segmenting the finite element grid when the position of the observation point is not on the finite element grid node, a set of finite element interpolation function methods using isoparametric element shape function as interpolation function were developed for interpolating the hydraulic head of the interpolation point. Accoding to these methods, the discrimination method of the topological relationship between the point and the plane/space unit is given respectively to obtain the attribution unit of the interpolation point. Moreover, the improved differential evolution algorithm is used to reversely find the local coordinates corresponding to the interpolation point, so that the hydraulic head of the observation point can be directly interpolated by the unit shape function. The interpolation performance of this method was tested using two dimensional and three dimensional examples. The results show that the calculation results are reliable and the calculation time is in the order of milliseconds.