Abstract: For analyzing non-proportional damping structure, the approximate method, modal strain energy method, exact method, and state equation method are introduced. The approximate solution and exact solution of multiple-degree-of-freedom model with Maxwell dampers in frequency domain and time domain are compared; then the accuracy and applicability of the modal strain energy method are investigated. The results of numerical examples indicate that the accuracy of modal strain energy method is acceptable in a certain condition and this method can be applied to engineering practice.