Due to the lack of research on the dynamical response of the inerter system based on non-stationary seismic excitation，an analytical solution for the time-varying variance of the dynamical response of a multi-degree-of-freedom energy-consuming structure with series-parallel layout Ⅰ inerter system （SPIS-Ⅰ） is proposed. According to the constitutive relationship of the SPIS-Ⅰ，the dynamic equations of the energy dissipation structure，and the impulsive non-stationary seismic excitation，we decouple the inertial energy dissipation structure into first-order systems using complex modal analysis and the virtual excitation method. It is convenient to obtain the unified solution of the structural response quantities such as displacement，velocity，inter-story shear force，etc. The quadratic decomposition method is used to transform the time-varying power spectral density function of the unified solution into a linear combination of the complex modal eigenvalues of the inertial-capacitated energy-consuming structure，the modal coefficients，the time-varying modal strength coefficients，and the quadratic product containing the squared term of the circular frequency. On this basis，an analytical solution for the time-varying variance of the response of the energy-consuming structure under non-stationary seismic excitation is derived by utilizing the characteristics that the non-stationary modes spectral moments have an analytical solution in the infinite integration interval. The accuracy of the proposed dynamic response power spectrum and time-varying variance is verified by using the sudden white noise excitation to analyze the dynamic response of the structure. At the same time，the dynamic response of the frame structure based on the sudden Kanai-Tajimi model is studied，and the influence of the parameters of the inertial system on the damping effect is analyzed. The proposed method can be applied to analyze the seismic response of linear structures under other non-stationary modulation functions.