Complementary variational principles are applied to the analysis and computation of three-dimensional electrostatic field. The computation errors of field with the potential differential method (PDM)and complementary variational method (CVM) are all shown and compared. By using CVM.we can obtain a field accuracy much better han PDM. To compare the accuracy of numerical methods more easily.a three-dimensional field model which may be expressed analytically without using an infinite series is constructed. This model can be used as a ' Benchmark' of numerical method for three-dimensional electrostatic field.