In the excavation of West Market, there discovered two types of drain canals: earth-tamping and brick-laying. The earth-tamping canal was earlier which about 0.3m in width. The canal wall was covered with board which was supported by the wooden pole outside the board to prevent collapse. The brick-laying canal was later which was rebuilt or replaced from the early canal. The canal was 1.1m in width at the bottom, 1.2m remained in width at the top and 0.65m in depth. The Chang’an City of Tang Dynasty was surrounded by eight rivers; to introduce the water into West Market solved not only the transportation but also the water supply which became one of the basic conditions for the prosperity of West Market.