Abstract: After An-Shi Rebellion the North Capital(Taiyuan) was the north gate of the dynasty. Taiyuan had become the center of the northern defense line. The commander made his decisions based about the border using the city as the core from which to manage the entire line of defense. When the nomads groups invaded the south, the Taiyuan yin was required to be a strong leader. The attitude of the imperial court to these various nomad polities changed and different policies were chosen by different individuals during their time as Taiyuan yin. This article studies the complex relations of various groups in the north during the middle and late Tang Dynasty. Furthermore, the groups around Taiyuan expanded further southward to accumulate their own strength, and outside-in promoted the chaos of the Five Dynasties.