Research conclusions ： The traction substation alternate phase，which is different phase alternating connect to the power system，is adopted at large in railway now. The terminal sign and connection of traction transformer with single - phase connection, single - phase V, v connection, three - phase V, v connection, three - phase YN, d11 connection, three 一 phase - two - phase balance connection and three - phase V, x connection have been discussed and summarized, and the phase alternating connection has been analysed and studied, and commended project about terminal sign and connection of transformer in PDL autotransformer feeding system has been proposed in this paper. Adopting this project can simplify the connection and avoid the cross connection of the high voltage side of the traction transformers, and can simplify the layout of the traction substation and investment is saved.