Abstract: The model condensation method is widely used in the static and dynamic analysis of structures. With the model reduction, the calculation scale of structural static and dynamic equations can be effectively reduced, with the calculation time and cost saved, and to obtain a reduced finite element model matching the experimental measurement freedoms. A new IRS (improved reduced system) method is proposed in this paper by considering the second-order inertia, to effectively improve the calculation accuracy. Compared with the IIRS (iterated IRS), the proposed second-order IRS method can be used to build a more accurate reduced model with smaller calculation amount. Taking a truss structure and a frame structure as examples, the proposed second-order IRS method is verified, and the calculation results are compared with the exact solution, the Guyan condensation solution, the IRS solution and the IIRS solution. It is shown that the proposed method enjoys the best calculation accuracy, with a good engineering application prospect.