The invention relates to a hybrid DC circuit breaker system and a fault line detection method for circuit breaker switching on. The transfer branch of the hybrid DC circuit breaker has a characteristic that multistage modular units are connected in series. By a manner that the transfer branch is switched on hierarchically, the current in an arrester is determined to determine whether the switchingis in a fault line. Compared with the prior art, the method can relatively reduce the capacity of the arrester and reduce the economic cost of the hybrid DC circuit breaker while detecting the line fault timely. The method is especially suitable for determine whether the reclosing is in the fault line during the reclosing of the DC circuit breaker, avoids the large short-circuit current generatedby the direct closing in the fault line, effectively avoids the damage to other DC equipment caused by the expansion of a fault range.