Deficiencies existing in China code of buildings aseismic design for long-period range of an acceleration response spectrum were analyzed. According to the definition of site classification and the seismic ground motion feature period Tg in the seismic ground motion parameters zonation map of China, 7274 ground motion records selected from 1146 real strong earthquake events with the rank  5.0 were divided into groups. Based on the random vibration theory, the statistical energy density distribution and acceleration response spectrum were obtained with the conversion relation among seismic ground motion’s Fourier amplitude spectrum, power spectrum and designed response spectrum. The attenuation laws of energy density distribution and acceleration response spectrum were studied. Through regression analysis, the reasonable descending form and slope value in long-period range of the designed response spectrum were deduced under different site conditions. The results provided a reference for the code modification.