Abstract: A new method which combines both time domain and frequency domain identification for identifying linear timeinvariant dynamic systems is presented in this paper. When the transfer function of a high order system is given this can be called a Padé approximation with multi-adjustable parameters. In this method, parameters of the "quasi-equivalent" model of the dynamic system are estimated by identifying the first few of the leading .low order time moments and the corresponding frequency response data that characterizes the dominant dynamic performance of the high order system. In addition, the precision of the calculation of time moments is effectively improved through the method of "extrapolation function". Practical examples of identification and reduction illustrate that this is a simple and convenient method with high precision and can be satisfactorily used in engineering modelling and design.