With the development of multi-electric aircraft, the researches on the structure improvement and fault diagnosis of aviation transformer rectifier units (TRUs) are increasing, but research on the characteristics and effects of typical fault modes of different types of TRUs is lacked. Based on the analysis of TRU structures, the failure mode and effects analysis (FMEA) and fault tree analysis (FTA) of TRUs are studied to determine typical failure modes of TRUs. Then, test prototypes of various TRUs are established by Simulink, and the electrical signal characteristics of TRUs under different fault modes are analyzed and compared; the detailed fault modes of 8 TRUs are defined. The results show that the hard faults of TRUs have great impact on the system, and the high-pulse TRUs have better stability under fault modes and lower reliability, which provides a basis for fault diagnosis and fault location research of TRUs.