Towards the end of the Eastern Han Dynasty, the wide north of the country was ruled by Cao Cao with the Emperor in his hand under duress. In 220 AD, his son Cao Pi finally dethroned Emperor Xian of the Han Dynasty, and proclaimed himself as emperor, with the dynastic title as Wei and the reign motto as Yingchu. It is referred to as Cao Wei (220-265 AD) in history. Altogether the Wei Dynasty had five emperors, and the dynasty existed for 46 years. In 232 AD, in the sixth year during Taihe period of Emperor Ming of the Wei Dynasty, the emperor conferred the title of Prince to his marquises to rule the commanderies as princedoms. Thus, Yan Commandery was transformed to the Princedom of Yan with Cao Yu as Prince of Yan. In the fifth year during the period of Yingchu (224 AD), Cao Min was enfeoffed Prince of Fanyang. At the beginning of Cao Wei Dynasty, constant changes took place in the administrative regionalization of the commanderies and counties in You Region. The Rgeion of You had a very wide domain, east of which reached Liaodong and the area where the present Korean peninsula stands. Textual research shows the Region of You during the period of Cao Wei had four princedoms including Yan, Fanyang, Yuyang and Shanggu. The present Beijing area covered four counties of the Yan Princedom: Ji, Changping, Jundu, and Guangyang. The capital of You Region was Ji city (the present southwest of Beijing city proper). And some other counties of the Cao Wei Dynasty are now parts of the present Beijing. They are Liangxiang of Fanyang Commandery with Zhuo County as its capital, and Yuyang, Anle and Lu of Yuyang Commandery with Yuyang as its capital (the present Huairou area), Juyong County of Shanggu Commandery with Juyong as its capital (the present Yanqing).

Along with the end of the war chaos in the last years of the Eastern Han Dynasty and the short reunion of northern China during the period of the Cao Wei Dynasty, a comparatively stable social environment appeared in the present Beijing area, and gradually the economy was restored. In 250 AD, Liu Jing, the general of Zhenbei of the Cao Wei Dynasty, did field investigations in search for riverheads and made layouts himself to build the Liling Weir near the Liang Hill (the present Shijingshan in Beijing), dig Chexiang Trench to carry water from the Luo River to irrigate the fields around the city of Ji. This was the earliest large-scale manual irrigation works in the recorded history of Beijing and also was of great significance in the history of water conservancy in Beijing.

In 262 AD, Emperor Yuan of the Wei Dynasty dispatched Fan Chen to You Region to alter the Liling Weir by rebuilding the floodgate and extending the watercourse of the Gaoliang River so as to enlarge the irrigation area. In order to speed up the development of agriculture, the Cao Wei regime made an enormous effort in the area of You Region to open up wasteland as a state policy. It was carried out in two types of practice. One is to organize garrison troops to cultivate while the other is to attract and encourage refugee peasants to open up wasteland and grow food grain. The former is called military colony land, and the latter civil colony land. After tens of years�� operation, there was an increase in arable land and population as well. As a result, the social economy was restored and developed to a certain degree.

During this period, the power of the regime in the Central Plains represented by the Cao Wei Dynasty was comparatively stronger, and Ji city of You Region became a city of military importance. First, it was the northern barrier for the Central Plains. During this period, there were troubles on the northern border caused by the minorities of Xianbei and Wuwan, etc. The Cao Wei Dynasty established a command (Xiaowei ����) to defend against the tribes of Xianbei and Wuwan, and to settle down the refugee peasants. Second, Ji city also served to be the base for the Cao Wei regime to enter and figure for Liaodong. In 237 AD, Emperor Ming of the Wei Dynasty sent General Wu Qiujian on a punitive expedition to fight against Gongsun Yuan, who set up a separatist regime in Liaodong. In 244 AD, the general was sent to invade Gaogouli (the present North Korea). All these military activities were carried out, without exception, with Ji as the base.