Reasonable seismic design input energy spectrum is important basis for promoting the engineering application of energy method. Based on 259 seismic records on 3 site conditions, the input energy spectrum of elastic single degree of freedom (SDOF) systems were studied. Using the normalization method, the elastic input energy spectrum are characterized as the product of the maximum of input energy spectrum (E_Imax) and the normalized input energy spectrum. According to correlation analysis, influence factors and estimation method of the maximum of input energy spectrum are given; based on characteristics of the normalized input energy spectrum on different site conditions, the simplified normalized input energy spectrum is established by multiple fitting, and the calculation method of design input energy spectrum of elastic SDOF systems with the damping ratio of 0.05 is proposed and verified.