Abstract: In order to deeply look into the relationships between occurrence rule and meteorological factors of fog in Shenyang, and then to accurately carry out fog forecasting, based on the observation data of fog from 1951 to 2011, the occurrence rules and the relationships were investigated. The results showed that: the annual total of fog was decreasing with a climate abrupt change happened around the late of 1970s when the decrease trend began to be exacerbated and reached very significant level in the earlier 1980s and then continued until 2011. There was a remarkable seasonal pattern for the fog in Shenyang with the larger odds from August to next year January than from February to June as well as the biggest and the smallest odds in September and June, respectively. For the diurnal time scale, the fog in Shenyang appeared most frequently from 2 to 8 o’clock and last more often less than 6 h and most frequent 1-2 h and no longer than 21 h. The optimal weather condition that trigger the emergence of the fog in winter was 1.0- 2.5 m/s wind speed, 70%- 90% relative humidity, suitable temperature and less low clouds.