As a connection region between Arctic and North Atlantic oceans, the Nordic seas play a critical role in global climate system. The density waters overflow through Greenland-Scotland Ridge from the Nordic seas, as the main source of the North Atlantic Deep Water (NADW), which plays a key role in global ocean conveyor. The causes and processes, which give some instruction of the overflow variation are still uncertain. Based on a review of current and historical research results of modern Nordic seas overflows, hydrological and flux characteristics and variation features of overflows through three channels, which are Faroe-Shetland Channel, Iceland-Faroe Ridge and Denmark Strait, from Nordic sea were addressed separately. The origins of overflows water and factors and physical processes that may have impact on the three overflows were also analyzed separately. Intense mixing in overflow through Faroe-Shetland Channel was discussed. At last, the changing mechanism of the whole overflow from Nordic seas and relationships among overflows through different channels were summed up. The aim of this paper is to give some instructions and research directions to the internal readers.