Side collision is one of the important risks of train operation in the turnout area of urban rail transit. The flank protection of the signal system is usually accomplished by the Automatic Train Protection system and the interlocking system together. This paper mainly discusses the existing flank protection of the Automatic Train Protection subsystem and its problems, optimizes its design, and extends the design to the double-location parking line of the mainline and Communication-Based Train Control depot/parking yard.