Abstract: In order to improve the forecast accuracy of fog in southwest Shandong, by using conventional observation data, numerical prediction model products and satellite cloud image, the causes of the heavy fog in March 17, 2012 at Jining City was analyzed .The results showed that: (1) the height and strength of inversion layer had close relationship with the concentration of fog, the weak cold and warm advection might be beneficial to produced fog; (2) The heavy fog showed significant influence from the change of relative humidity on 1000 hPa, the fog would occur if the surface layer had inversion and relative humidity≥80% on 1000 hPa; (3) The dew-point deficit in land (T-Td)≤2℃ could easily produce the heavy fog; (4) The environmental condition of forming fog provided by numerical forecasting pattern products; (5) Infrared images and visible cloud picture were helpful to prediction the formation, development, dissipation of fog.