Abstract: High-frequency transformers (HFTs) are key devices responsible for electrical isolation and voltage conversion in power electronic conversion systems. Due to the compact structure of high-power HFTs, the composite or cast insulation of HFTs is subjected to severe electrical, thermal, and mechanical stresses, which challenges insulation design. In this review, the application background and structural composition of HFTs are first revised. The voltage stress on the main insulation and longitudinal insulation of HFTs is discussed. Local dielectric loss issue of HFT is addressed. The contradiction between power density and insulation reliability is expounded from the perspective of optimal design of HFTs. The mechanical stresses on insulation in HFTs are clarified. Then, the research status of HFT insulation design in recent years is extracted, outlining the commonly used insulation materials, insulation forms, and insulation level. The influence of insulation design on transformer volume, electric field distribution and parasitic parameters is detailed, and the melioration direction of insulation design process is proposed. The dielectric loss, partial discharge and space charge characteristics of polymer insulation under high-frequency voltage are the focus of current research, and these factors are usually coupled with each other. The influences of high-frequency square waveform parameters and temperature on partial discharge, insulation life, space charge and electrical treeing are analyzed and summarized, and the margin in the research field is pointed out. Finally, the stress characteristics, insulation design, and researches on insulation issues of HFTs are summarized.