The hybrid multi-terminal DC transmission system realizes operation mode conversion，fault isolation and protection functions through the cooperation of various DC circuit breakers，and studying the action characteristics of DC circuit breakers after various faults is of great significance for discovering potential risks of the system. This paper constructs the Kunliulong （Kunbei-Liuzhou-Longmen） UHV three-terminal hybrid DC system and DC circuit breaker PSCAD/EMTDC simulation model，traverses the typical faults on the AC side，inner station and DC side of the system，and analyzes the operating characteristics of the circuit breaker in different fault recovery processes. The most serious fault state of each circuit breaker is obtained. The research results will help to find the weak parts of the system，and provide reference and basis for the system operation and maintenance and dispatching.