The reasons of the constant maintenance of the advection fog in Beijing area from December 17 to 19 in 1997,was synopolically analyzed.It is suggested that under the macroscale condition of the weak convergence which is formed by the relatively stable topographic trough in north China,saturated air is firm in a special terrainBeijing gulf which is below 239.4 m,due to the scarce inversion lid(r=5.84℃/100m);and that the surface mesoscale convergence line and the miniscale convergence line below 200m produced by the fluctuation of the wind on the timeheight section diagram result in the convergence lift.Thus helped the saturated air constantly being supplied and gathering in plains and valleys.