The invention discloses a construction method of a city underground excavation section over crossing an existing line. The construction method comprises the following steps that firstly, a pipe curtain is constructed above the existing line, and the inner part of the pipe curtain is used for constructing the underground excavation section; and then prestressed anchor cables are constructed on thetwo sides of the pipe curtain, and the anchor cables extend to the lower part of the pipe curtain, so that the existing line can be pulled by the anchor cables when the existing line is pre-bulged. The aim is to provide the construction method of the city underground excavation section over crossing the existing line aiming at the problem that an existing traffic line structure cannot be preventedfrom bulging in existing pipe curtain method construction. According to the scheme, the position of the pipe curtain in a stratum can be fixed, so that a load above the existing line is kept unchanged, the structure of the existing line is prevented from bulging, and thus the operation safety of the existing traffic line is ensured.