Abstract: Aiming at the problem of frequently changing running state, high fault probability and lack of operator support means for marine nuclear power plant (NPP), a fault diagnosis method based on characteristic event sequence was proposed. In the arithmetic, the characteristic event sequence was defined to identify fault characters, and a standard characteristic event sequence chart was built according to NPP record running data. When the system ran out of the way, a characteristic event sequence was constructed from current running state data, and the accident grades and rough positions were diagnosed by comparing current event sequence with standard characteristic event sequence. Validated by experiments, the arithmetic can fix accident grade and rough position, and it has lots of excellences comparing to the traditional data driving arithmetic, such as easily casting back, explicable, and therefore it is practically useful.