Based on the data of GTSI digital radiosonde, NCEP reanalysis data and conventional observation, synoptic analysis of the uncommon long-drawn fog over Huabei plain in the winter of 2004 is made. The result shows that the temperature of layer close to the surface was lowered by negative vorticity advection, cold advection and weak convergence ascending movement below 900hPa, and the temperature of the layer between 900hPa and 500hPa was raised by the positive vorticity advection, warm advection and divergence descending movement, thus the inversion layer is formed in the interface of ascending and descending movement. The height and intensity of the inversion layer could affect on the formation and status of the fog. Radiation cooling during the night and gentle breeze below 950hPa layer are dynamic factors for the formation of fog. Under some synoptic situations, the easterly and southeasterly shoreward wind over Huaibei plain transports the water vapor from northern sea surface to cold land surface layer, and results in the saturation in fog boundary layer.