The advantage of 18pulse autotransformer rectifier unit(ATRU) is low input current harmonic content,output voltage ripple and transformer equivalent capacity.But the output voltage can not satisfy the requirements of the aviation low voltage DC system.This paper based on 18pulse autotransformer rectifier,designed buck circuit,kept the advantage of original performance and achieved the 28 V DC output.The paper introduced the phase shifting principle and winding structure of 18pulse autotransformer rectifier firstly,then analysed the output voltage characteristic.According to the need of reducing voltage introduced the buck circuit and it’s control circuit.In addition,designed parameters of main circuit and control circuit.Finally,the system was established and analysed in Saber simulation environment, verified the system had a good input and output performance.