In this paper the reports of seismic safety evaluation of the typical sites are used to study the seismic effect of medium-stiff soil site in Beijing area.Firstly

the seismic amplification effect is statistically analyzed from the reports of seismic safety evaluation.Secondly

based on the bore data of 201 typical engineering sites

the seismic response analysis of soil layer is carried out

and the seismic amplification effects for various seismic inputs are analyzed.Our results indicate that the seismic amplification effect of medium-stiff soil site is remarkable in the study area

the seismic peak ground accderation factor Ka of the site is larger than that recommended in GB18306-2001 "Seismic Ground Motion Parameter Zonation Map of China"

especially for large earthquake.Therefore

if the adjustment factor in GB18306-2001 "Seismic Ground Motion Parameter Zonation Map of China" is used directly

the seismic amplification effect of medium-stiff soil site may be underestimated

and the seismic fortification criterion may be played down in Beijing area.Finally

we point out that the seismic peak ground accderation factors Ka of the site clearly has territorial

and the seismic factor must be determined reasonably on the basis of the detailed research results of the seismic effect of the site.