The Guanganmen area is the locality of the ancient Jicheng City (the earliest city site in Beijing). Based on the geomorphological analysis and the several kinds of experiments for the samples collected from the geotechnical profile in the construction site of Guangyi Building in Guanganmen area, the reconstructions of palaeogeomology in this area were conducted. The results show that the location of the ancient Jicheng City was just the highland, which developed since the Holocene, beside the river, where the channel of Yongding River has been passed through about 22 000 years ago.