Compared with the Tang and Song Dynasties, the territory of the Yuan Dynasty was wider, and the degree of openness to the outside world was greater. However, due to years of external warfare and failure, as well as the chaos in coastal defense and the control of sea merchants by bureaucratic aristocrats, the Yuan Dynasty government increasingly felt the destabilizing factor brought about by the opening of ports. As a result, Emperor Shizu of the Yuan Dynasty, Kublai Khan, carried out four “Maritime Prohibitions” in succession. The policy of opening ports since the Han Dynasty was replaced with the situation of “banned sea merchants,” so stopping the trade ships and addressing coastal defense became a priority for the Yuan government during the Yuan Dynasty.