本文主旨，即探討北平行都司的設置、內徙及其影響，並分析明代北邊邊防敗壞的根本原因。全文分為三個部分。首先，討論洪武朝的國防策略與邊防布置，可知北平行都司在明初北邊邊防的地位，相當重要。其次，說明「靖難」對北平行都司的影響，可知北平行都司內徙的真正原因，即燕王朱棣發起「靖難」時，寧王響應，盡調北平行都司的兵力，支助燕王，卻讓大寧一地殘破不堪，無法再作為軍事重鎮，只好內徙。第三，分析北平行都司內徙的連鎖效應，可知北平行都司內徙保定後，大寧不再設防，兀良哈部得佔據大寧，讓遼東、宣府無法聯絡，也讓北京以北、西、東三邊防衛出現漏洞。此後，蒙古、女真得乘隙而入，不時騷擾，讓明政府顧此失彼，疲於奔命，也使九邊邊鎮左支右絀，難以協防。最終，明帝國的長城與邊鎮，仍檔不住八旗鐵騎，明朝遂亡於女真之手。

The proposed research is aimed to discuss the influences of the establishment of Beiping Xingdusi and its moving inward from Daning to Baoding in later years, and to reach a better understanding of the fundamental causes that corrupted the defense of Northern frontier in Ming Dynasty. This research consists of three sections. The first section of the paper, by looking into the defense strategy and border layout during the reign of Hongwu, shows the significant role Beiping xingdusi played in the defense of Northern frontier in early Ming Dynasty. The second section analyzes the impacts of the Jingnan Campaign (靖難) on Beiping xingdusi and the truth of relocating Beiping xingdusi in Baoding after the campaign. As what is observed, during the coup d'etat initiated by Zhu Di, the prince of Yan (燕王朱棣), in response to the call of Zhu di, the prince of Ning(寧王) sent all the forces of Beiping xingdusi to support him, leaving Daning unguarded. As a result, Daning was so severely damaged that it could not remain a strategic post after the campaign, and consequently resulted in the relocation of Beiping xingdusi. The last section deals with a series of chain reactions triggered by the relocation. Once the government moved Beiping xingdusi to Baoding, Wuliang habu(兀良哈部) took over Daning, which was then no longer fortified, and broke the communication between Liaodong (遼東) and Xuanfu (宣府). inevitably causing the defenses in north as well as in east and west of Peking to fall vulnerable to external forces. Henceforth, the Mongolians and the Manchus, recognizing the weakness of Ming authority in the north, invaded and harassed the border even more frequently, and made the Ming government and the nine border cities too exhausted to cope with the difficult situation. In the end, neither the Great Wall nor the border cities stood the attacks of the Eight Banner cavalry; the fate of the dynasty ended at the hand of the Manchus.