The rapid urbanization produces more and more long-period structures

and process requires design spectrum improvement for long-period structures

meanwhile

the effect of far-field ground motion has to be considered in seismic design.However

as a result of lack of far-field ground motion records

previous studies on far-field ground motion characteristics are insufficient.In this paper

a detailed analysis among station logs in Wenchuan earthquake was carried out

and the relations between far-field ground motion and influencing factors in terms of propagation distance and site conditions were investigated.Results show that:（1）along with the increase of propagation distance

the low frequency component of ground motion increases

so that buildings with natural period greater than 0.5 second on the bedrock sites and those with natural period within about 0.5—2 second and 3—6 second on the soil sites will undergo obvious amplification effect.（2）The statistical response spectrum of far-field ClassⅡsite is significantly higher than design response spectra in period 1—4.5 second

and the characteristic period of Class Ⅲsite in statistical response spectrum is longer than that in design response spectra.Therefore

the design response spectra have been underestimated in the component of long period

which should be appropriately enlarged.