Abstract: An autotransformer (AT) traction power supply system equipped with 2×27.5 kV outdoor modularized electric apparatus comprises a traction substation located beside an electrified railway, which connects with an up and a down line traction network at output terminal, a parallel apparatus located in the middle and the end of a feeder in the up and down line traction network, which functions as parallel power supply operation, and an over-zone apparatus located around a neutral section insulator between feeders of the up and down line traction network of two adjacent traction substations, which functions as over-zone power supply operation. The parallel apparatus connected with an AT equipment is a modularized electric apparatus which comprises four parallel power supply units. The AT equipment comprises four integrated AT substations which include the first AT, the second AT and an AT modularized electric apparatus.