According to the 《史記.十二諸侯年表》, the Zhou dynasty held hundreds of vassal states, while the Qi and Yan were the countries which successfully last for a long period. Though Yan's longevity was a bit shorter than Qi, Qi was usurped by Tian Shi in 386B.C, while Yan kept maintaining its imperial power and family for 42 generations. Yan used to be an unknown and remote country before Zhan Guo(戰國) era, it became one of the seven strongest countries after the King of Yan Zhao made widely recruitment and reformation. It is interesting to know for what reasons that a poor and remote country could survive for such a long time, but not being ruined by other competitive opponents. This paper will probe into the reasons of Yan's longevity from the aspects of geographical environment, political, economic and military.