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The spatiotemporal variation of fog reflects the complex interactions among fog, boundary layer thermodynam9 ics and synoptic systems. Previous studies revealed that fog can present fast spatial propagation feature and attribute it to boundary layer low-level jet (BLLJ), but the effect of BLLJ on fog propagation is not quantitatively understood. Here we analyze a large-scale fog event in Jiangsu, China from 20 to 21 January 2020. Satellite retrievals show that fog propagates from southeast coastal area to northwest inland with the speed of 9.6 m/s, which is three times larger than the ground wind speeds. The ground meteorologies are insufficient to explain the fog fast propagation, which is further investigated by WRF simulations. The fog fast propagation could be attributed to the BLLJ occurring between 50 and 500 m, because the wind speeds (10 m/s) and directions (southeast) of BLLJ core are consistent with fog propagation. Through sensitive ex16 periments and process analysis, three possible mechanisms of BLLJ are revealed: 1) The abundant oceanic moisture is transported inland, increasing the humidity of boundary layer and promoting condensation; 2) The oceanic warm air is transported inland, enhancing the inversion layer and favouring moisture accumulation; 3) The moisture advection proba19 bly promotes upper-level fog formation, and later it subsides to ground by turbulent mixing of fog droplets. The fog prop20 agation speed would decrease notably by 6.4m/s (66%) if the BLLJ-related moisture and warm advections are turned off. [ABSTRACT FROM AUTHOR]