Based on the observed daily fog data from 52 meteorological stations in southern China and ERA5 reanalysis data from 1979 to 2016, effects of ENSO on the interannual variability of winter fog days over southern China and the interdecadal variation of the relationship between them are investigated using the EOF analysis and other statistical methods. Results are shown as follows: (1) The winter fog days over southern China (WFDSC) exhibit a whole-region consistent pattern and display evident interannual variability and significant increasing trend. The WFDSC is closely related to the weakened northerly wind, more stable atmosphere and the decreasing T-Td near surface, which implies that the water vapor in air is increasing, providing favorable meteorological background fields for the maintenance and development of fog over southern China. (2) There is a close relationship between ENSO and the interannual variability of WFDSC, which mainly depends on heavy fog days rather than light fog days. During El Nino winters, an anomalous anticyclone tends to occur over the Philippine Sea, and the warm moist anomaly southwesterlies on the western flank of this anticyclone brings abundant water vapor to southern China, conducive to the fog formation. Roughly opposite changes of fog days occur during La Nina winters. (3) The interannual ENSO-WFDSC relationship exhibits an obvious interdecadal change around 1996, with a low correlation during the period of 1979-1996 but a high correlation after 1997. A further analysis shows that this change in the relationship might be modulated by the Atlantic Multidecadal Oscillation (AMO).