The　current　acceleration　seismic　design　spectra　has　the　disvatages　including　that　its　precision　and　the　statistics　features　in　different　period　stage　have　obvious　differences,　and　the　spectrum　characteristics　of　the　low-order　spectrum　based　on　the　artificial　signals　mismatch　the　real　statistics　characteristics.　The　unified　smooth　spectrum　which　adopts　unique　function　was　presented　based　on　the　linear　modle　of　displacement　spectrum,　and　a　new　ground　motion　simulation　method　considering　lower-order　spectrum　was　proposed.　The　analysis　results　indicate　that　the　smooth　spectrum　has　the　advantages　such　as　simple　form,　high　accuracy　for　all　the　period　stages.　The　simulated　earthquake　signal　fulfills　the　spectrum　characteristic　requirement　for　acceleration　spectrum,　velocity　spectrum　and　displacement　spectrum.　The　smooth　spectrum　has　uniform　characteristics　including　function　form,　the　spectrum　characteristics　on　multiple　orders,　elasto　plasticity　and　multiple　dimensions,　and　it　is　suitable　for　dynamic　analysis　and　ground　motion　simulation.　©　2016,　Editorial　Board　of　Journal　of　Harbin　Institute　of　Technology.　All　right　reserved.