The utility model provides an ultra-high voltage single phase autotransformer, belonging to the transformer technical field and being used for solving the low voltage fluctuation problem. The technical proposal is as follows: the ultra-high voltage single phase autotransformer consists of an autotransformer body, a regulating transformer and a low voltage compensation transformer, wherein the autotransformer body consists of a low voltage winding, a common winding and a series-wound winding, the regulating transformer consists of an excitation winding and a regulating winding, the low voltage compensation transformer consists of a low voltage excitation winding and a compensation winding, and the connecting relation of each winding is as follows: the regulating winding and the common winding as well as the series-wound winding are connected in series and are connected with an input voltage, two ends of the regulating winding and the common winding are connected with an output circuit, the low voltage excitation winding and the regulating winding are connected in parallel, and the low voltage winding is connected in parallel with the excitation winding and is connected in series with the compensation winding as well as is connected with a compensation network. The ultra-high voltage single phase autotransformer not only has simple structure and low cost, but also has small fluctuation of the low voltage and high insulating reliability, and meets the requirements of various users.