A capacity adjustable traction autotransformer is applied to an AT power supply mode traction power supply system. The working capacity of the autotransformer can be adjusted according to the traffic capacity, the traffic volume and the running mode of the autotransformer, the running condition of the transformer is improved, the capacity utilization rate is increased, and the electric energy loss is reduced. The capacity adjustable traction autotransformer comprises a main core limb and an auxiliary core limb. A main high-voltage winding and a main low-voltage winding are arranged outside the main core limb, the access voltage of the main low-voltage winding is 25 kV, and the access voltage of the main high-voltage winding is 50 kV. An auxiliary high-voltage winding and an auxiliary low-voltage winding are arranged outside the auxiliary core limb, and a high and low voltage linkage tapping switch is arranged between the auxiliary low-voltage winding and the auxiliary high-voltage winding. The main high-voltage winding and the auxiliary high-voltage winding are connected with an overhead line system and a positive feeder after being connected in parallel, and the main low-voltage winding and the auxiliary low-voltage winding are connected with the positive feeder and a steel rail after being connected in parallel.