Based on conventional meteorological data of Shipu Station from 1971 to 2011 and NCEP reanalysis data, characteristics of atmospheric environmental elements and weather backgrounds during the sea fog generation in Ningbo was analyzed. The results show that in most cases, the sea fog occurs in the period from April to June, mainly between 23:00 BT and the next 11:00 BT (especially from 23:00 BT to the next 02:00 BT), usually dissipating after 11:00 BT every day. The southwest and northeast winds in 2-3 grades are greatly conducive to the occurrence of sea fog. When relative humidity is 90%-95% and station pressure is 990-1002 hPa, frequency of sea fog increases with the increase of humidity and the decrease of pressure. However, when it comes to severe dense fog (visibility ≤50 m), relative humidity is 92%-95% and station pressure is 990-999 hPa, and the frequency increases with the increase of humidity and pressure. Weather systems including Jianghuai cyclone, Meiyu front and cold air can influence the occurrence and development of sea fog. Frontal fog usually occurs 2-3 days before weather system transits the area and when the upper air in the local area is influenced by southwest warm wet air. The upper air is in front of the southwest vortex when advection fog occurs. Under this condition, temperature difference between sea and air is 0-1.5℃ and -0.5-0℃ between dew point and sea underlying surfaces. Severe dense fog or dense fog occur under stable atmospheric stratification and in long standing and thick warm wet air flows, and meanwhile, cold ocean current from north continuously pours into the sea near Ningbo. Besides, sea fog intensity is related to warm advection and cold ocean current to some extent. All the above discussed perform an important role for the forecasting of sea fog and its intensity in Ningbo.