Interaction of AC and DC system in long-distance， high-capacity， ultra-high voltage AC/DC hybrid system is very complicated. Based on the analysis of operational examples in China Southern Power Grid， the effects of DC control system during AC system fault are discussed first. Then some functions that maybe misoperate during the AC system fault， including the DC protection and monitoring function， converter transformers protection and auxiliary transformers， are analyzed. The improvement suggestions are given， which are not only valuable for improving the operation and maintenance capability and insuring the safe and reliable operation of HVDC system， but also provide useful reference to the design of HVDC transmission projects and the cooperation of AC/DC system in China in the future.