Houhai is an integral part of Shichahai, composed of Qianhai, Houhai, and Xihai three water surfaces, in order to distinguish from the North Sea, the Middle Sea and the South China Sea "first three seas", it is called "back three seas". Houhai East from Di'anmen Outer Street, West to Xinjiekou Street, South to Ping'an Street, North to North Second Ring Road, the total area of 146.7 hectares, of which 34 hectares, green area 11.5 hectares. This is the ancient waters of the Yuan Dadu period 700 years ago in Beijing.