Abstract: The practical power spectrum density function (PSDF) of earthquake ground motion produced based on the response spectrum (RS) is very useful in engineering applications.The rationality analysis of the low-frequency stage of the practical PSDF based on several response spectrum of different codes is carried out on the base of two aspects,both rationality of the characteristics of PSDF value at zero frequency and the peak displacement of ground motion.Results show that the rationality of PSDF depends on that of RS.RS value at the long-period (low-frequency) stage must attenuate at the speed of T^-2 with increasing period T.Then descending stage of T^-2 is adopted to improve the RS curve of Chinese code GB50011—2010.An estimated value of 2 s is proposed as the starting period of the T^-2 descending stage.