Abstract: Fiber impurity particles account for more than 90% of transformer insulating oil particles. The fiber impurity particles suspended in the insulating oil are prone to agglomerate in the high voltage direct current(HVDC) electric field, which causes the electric field to be distorted and the endurance capability of the insulating oil to deteriorate. With the rapid development of HVDC power transmission project in China, the research on fiber impurity particles in the liquid insulation system of converter transformers has gradually become a new research hotspot. This paper summarizes the research status at home and abroad, discusses the research progress of fiber impurities in insulating oil, including the source and force of fiber impurity particles, law of movement and accumulation, discharge characteristics, as well as impurity bridge blocking technology and the influence of oil flow on fiber impurity. This paper also presents the prospects in the future development trend of fiber impurity particles in converter transformers. The summary of these research results may provide reference for the safe operation and structural improvement of converter transformers.