The Ancient City of Xihai Prefecture is located 0.5 li (1 li = 0.5 km) west of Haiyan County in Qinghai Province.

The well-preserved city site is 650 meters long from east to west and 600 meters wide from south to north, with wall remnants measuring four meters in height. The four gates are in perfect condition. And a granite-carved tiger was unearthed at the site.

According to the History of the Former Han Dynasty, Wang Mang (emperor of the Xin Dynasty from 8 to 23AD) set up a Xihai Prefecture in Qinghai that lasted through the Yongyuan reign (89-104) of the Eastern Han Dynasty (25-220). The Xihai Prefecture became the Haixi Prefecture, also known as Triangle City, during the Southern Liang (502-557) and Western Qin periods.

Various coins were found at the city site, including coin molds, five-zhu, a half-liang, 25-zhu Huobu and Shengzong Yuanbao (shoe-shaped gold or silver ingots). Also discovered in the city were tile fragments containing patterns of clouds or lotus flowers. The unearthed cultural relics which belonged to different dynasties of various periods indicate that the city had existed for a long time.