In terms of observed data and NCEP reanalyzed data, circulation background and d rycold activity features of the persistent snowfall in Shandong Peninsula duri ng December of 2005 were analyzed. The roles of drycold air in this event were researched by using the moist potential vorticity and frontogenesis theories. T he results suggested that the change of snowfall in Shandong Peninsula could be well reflected by the lowlevel humidity field, while it was closely associated with upperlevel drycold air. The uppertroposphere potential vorticity are a was corresponded with the low relative humidity area. Dry air came mainly from the northern part (high latitude) of uppertroposphere. The down and sou thstretching of potential vorticity area and low humidity area were correspond ed with the lowlevel moist symmetry instability area. The severe snowfall occurred under the common actions of the lifting of topographic and frontogenesis forcin g of lowlevel saturation moist air, as well as the releasing of moist symmetry instability energy. The incursion of drycold air was the trigger mechanism of the releasing of instability energy.