The convection under sea ice in winter Amundsen Gulf without solar heating is studied. The data we used are from Circumpolar Flaw Leads System Study project (CFL) during the time of polar night from November 2007 to the end of January 2008. It is shown that the vertical haline convection is the main character of the sea water in winter time in this region. The thickness of the mixed layer varies from meters to tens of meters. No static instability is found, i.e. the density of the water in mixed lyaer is lower than that of the water below. Unlike the convection others studied, the convection we observed is caused by continual released haline parcel. The temperature of the water in the mixed layer is almost at the freezing point, which is the result of the haline/density convection during the period of the ice formation. Below the mixed layer, there is a warm layer. But the temperature decreased with time during the observation period. We think, the main reason for it is the energy exchange with the water in mixed layer. According to the results, the salinity and thickness of the mixed layer coincide in early winter, while the thickness decreased in late winter with the salinity increasing. We also analyzed the temperature and salinity data from 3 stations which are coarsely at the same position and found that the salinity of the mixed layer increased and the temperature of the warm layer decreased during the observation period.