Because of simple structure, high reliability, electrical isolation between input and output, low total harmonic distortion of the input current content and the function of adjusting output voltage freely through changing the turn ratio, multi-pulse isolation transformer rectifier is widely used in the field of industry and aviation. This paper constructs the 18-pulse asymmetrical isolation transformer rectifier by the way of adding a triangle winding as primary side based on the traditional 18-pulse auto-transformer rectifier units(ATRU). The operation principle of the rectifier is analyzed as well as the output voltage and the total harmonic distortion of the input current. And through an equivalent model of the transformer, the impact of the external characteristics of output voltage by the equivalent resistance of the transformer winding and the AC side leakage inductance is discussed, thus providing the basis to further optimize the transformer design. Finally, 1 kW and 200 kW 18-pulse transformer rectifiers are fulfilled, and the results are compared.