Abstract: Aiming at the requirement for voltage regulation with a 18-pulse autotransformer-rectifier, a novel step-down 18-pulse topology was designed and optimized based on the advantage of present topologies. The primary extended winding of the step-down 18-pulse autotransformer-rectifier was connected with the winding tap of the phase shift secondary winding to reduce the transformer input voltage. Considering the influence caused by the position change of the connecting tap between the primary extended windings and secondary phase shift windings on the autotransformer performance, transformer voltage ratio and winding tap position were regarded as variables. A universal design formula was derived and simulation analysis was carried out to obtain the optimal connection tap position with the minimum equivalent capacity of the step-down autotransformer under the same voltage ratio. Finally, the rationality of the design is verified by experiments.