Abstract: The characteristics of the CBTC (communication based train control) system are introduced. The difficulties of using the CBTC system scheme in the reconstruction of existing line signaling system are analyzed. The characteristics of TACS (train autonomous circumambulate system) are introduced. The advantages of using the TACS scheme for existing line signaling system reconstruction are analyzed. It is suggested that TACS scheme should be preferred in the improvement of the existing signaling system of urban rail transit.