The dynamic impedance of pile groups in the frequency domain is computed by thin layer method. It is represented by the complex transfer function in the frequency domain as single input and single output system. The fitting of complex transfer function is proposed through the vector fitting method (VF method)

so as to obtain the lumped-parameter model for pile groups in layered ground. The dynamic impedance of partial-fraction expansion type can be obtained by VF method; it fits over the whole frequency range

which is easily converted to the Wu-Lee model. It is shown that the lumped-parameter model by the VF method is accurately to match the dynamic impedance of pile groups in layered ground. Due to its partial-fraction expansion type，the expression can be conveniently transferred into the Wu-Lee model; and the fitting is numerically stable. The number of terms for the partial-fraction expansion is big when the dynamic impedance for the high frequency is needed to fit. The reduced order method called the Routh method is introduced to make the order reduced

and it is shown the result for the reduction is good.