Abstract: The Southern Song Dynasty was based in Jiangnan，and the defense on the Yangtze River became a critical matter of national safety.The Southern Song Dynasty established a defense system against the Jin Dynasty incorporating the defense on the Yangtze River as the last and most crucial defense，the war zones of Chuan Shan， Jing Xiang， and the defense on the Huai River as the outer defense.An army specialized in the defense of the Yangtze River and a multi tiered system of fortification on the Yangtze River were also established. Southern Song Dynasty’s defense system on the Yangtze River was proven effective in the defense of the Jin Dynasty and the crackdown on the rebel forces pillaging on the river， but it was defective that the lower reaches were well guarded while the upper reaches were not， and it resulted in the breach of defense by the Mongolian forces at E’zhou，and consequently accelerated the demise of the Southern Song Dynasty.

Key words: the Southern Song Dynasty, defense system on the Yangtze River, structure