Abstract: Based on the current situation that rail transit lines face the need to replace equipment at the end of service life, and the newly built lines find it difficult to relieve the passenger flow pressure of the existing lines, the necessity is analyzed from the definition of existing line capacity expansion and reconstruction. Different types of transformation such as the upgrading of existing line stations, vehicle formation expansion, transportation capacity expansion, line adjustment, as well as comprehensive reconstruction and others are expounded with an analysis of the implementation key points.