In recent years, many catastrophic blackouts occurred in some countries show that the hidden failure in backup protection is the key factor which worsen the process of cascading failures in power grid. So it is important to ensure our power grid against cascading failure by searching hidden failures in backup protection and eliminate them in advance to avoid blackout. This paper proposes a measure to calculate the probability of hidden failure according to power flow transferring distribution after one or more transmission lines stop working. And a new concept RCTL (relative cluster of the transmission line) and the corresponding searching algorithm based on shortest path algorithm are proposed, the method to calculate relative factor of RCTL is also introduced. Taking CEPRI 36-bus system as an example, the power flow transferring is simulated and the simulation results show that the algorithm is accurate and effective.