Based on conventional observation data, NCEP reanalysis data, a massive dense fog process lasting from 31 December 2016 to 4 January 2017 in Eastern China are analyzed with the focus on propagation and extension mechanism. The results show that the dense fog process took cold anticyclone moving south on the evening of 1 January as a demarcation point. Before the cold anticyclone moved south, dense fog was limited to southern and central part of Hebei Province. After the cold anticyclone moved south, dense fog enhanced and spread widely, affecting many provinces including Jiangsu, Anhui, Henan, Hebei and Shandong, etc. The dense fog spread in a large range by cold anticyclone moving south, causing the lowlevel temperature inversion to be strengthened and the range of the inversion area to be expanded. The water vapor transport from the sea through Jiangsu, Anhui and Henan to Hebei provided sufficient water vapor for the wide spread of the dense fog.