Abstract: In the early Tang Dynasty, the emperor was so busy integrating the country that the operation of the Southwest was ignored. And Tang and Nan zhao had a harmonious relation. Up to the reign of Wu Ze-tian, the threat of To Bo became worse. And Tang had to unify Nan Zhao, thus adopting a policy of supporting the latter’s integrating its country. But, when Nan Zhao became stronger, centring on the struggle about Cuan’s army, there appeared the conflict between Tang and Nan Zhao, which gradually made complicated for the national relation in the area of the Southwest, and finally influenced the social stability and security of the Tang Dynasty.

Key words: Nan Zhao, To Bo, Cuan’s army, Tang Dynasty