The more-electric aircraft (MEA) is the major trend for airplanes in the next generation. Comparing with traditional airplanes, a significant increase of on-board electrical and electronic devices in MEAs has been recognized and resulted in new challenges for electrical power system (EPS) designers. The design of EPS essentially involves in extensive simulation work in order to ensure the availability, stability and performance of the EPS under all possible operation conditions. Due to the switching behavior of power electronic devices, it is very time-consuming and even impractical to simulate a large-scale EPS with some non-linear and time-varying models. The functional models in the dq0 frame have shown great performance under balanced conditions but these models become very time-consuming under unbalanced conditions, due to the second harmonics in d and q axes. The dynamic phasor (DP) technique has been proposed to solve that problem. The DP technique is naturally a frequency-domain analysis method and has been successfully applied in modeling electromagnetic machines, flexible AC transmission systems, etc. This paper aims to extend the DP technique to modeling an 18-pulse autotransformer rectifier unit (ATRU) for MEA power system studies. The developed DP model is based on a non-switching, time-domain model with the symmetry of the ATRU been considered. Comparison of transient and steady-state behaviors with those obtained from the detailed model in abc frame is presented to validate the derived DP ATRU model. The results display good accuracy of the DP model. Compared with DQ0 model, the DP model demonstrates similar efficiency under balanced conditions while better performance under unbalanced conditions. The developed DP model can also be conveniently connected with standard time-domain ABC models using a DP-ABC interface, which enables the DP models wider applications.