Abstract: In order to analyse alarm of fault components and fault information, this paper utilized the theory of MFM (multilevel flow model) to identify fault components in complex fault situation of primary coolant. This algorithm based on CDG (causal dependency graph) can perform depth-first search of alarm components, confirm the root cause of alarm component and the path of alarm propagation. Alarm analysis system based on MFM can be used as sub-system of operation support system to assistant operators to make decision.