On the west side of the county seat of Haiyan County of Haibei Tibetan Autonomous Prefecture, there is the relic of an ancient city, which enjoyed long history and large scale. That is the seat where Xihai Prefecture lied in Han Dynasty. The ancient city is located on luxuriant Jintan and Yintan Grassland, which is on the northeastern side of Qinghai Lake. The ancient city is close to the northern bank of Maxu River and embraced by mountains from south and north. On the east and west, there is the open prairie, which enjoys strategic importance in terrains. The city shows a shape of a square. It is 650 meters long from the east to the west and 600 meters wide from the north to the south. The preserved city wall is 4 meters high and the base of the wall is 10 meters wide. There are four city gates, each one in each direction. The gate is in the centre of the rampart. The relics of the architectures on the central axes of the city could still be seen.