The presented research proposal focuses on the approximation of higher-order (HO) multi-input multi-output (MIMO) interconnected power system model (IPSM) by employing systematic approach of proximity index value (PIV) based weight ascertainment method. The approximation of HO MIMO IPSM is accomplished by utilizing expansion parameters of HO MIMO IPSM and its desired lower-order model (LOM). The expansion parameters of HO MIMO IPSM and its LOM are exploited to construct the fitness-function. In the fitness-function, weights are assigned to achieve better LOM by improving the steady-state and transient responses with respect to HO MIMO IPSM. The weights of constructed fitness-function are determined with the help of PIV method. Then obtained weighted fitness-function is optimized by satisfying the two prime constraints i.e. matching of steady-state response and preservation of stability of IPSM. To demonstrate the efficacy of the proposed systematic approximation methodology, nineteenth order IPSM is approximated to LOM of second order IPSM. Additionally, specifications of time-domain and error-indices along with responses of the proposed LOM are compared with other LOMs obtained using different approximation approaches, with respect to HO IPSM to depict the effectiveness and applicability of the proposed approach.