To the active basis choice decoy state quantum key distribution systems with detector efficiency mismatch, we present a modified attack strategy, which is based on faked states attack, with quantum nondemolition measurement ability to restress the threat of detector efficiency mismatch. Considering that perfect quantum nondemolition measurement ability dose not exist in real life, we also propose a practical attack strategy using photon number resolving detectors. Theoretical analysis and

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... cal simulation results show that, without changing the channel, our attack strategies are serious threats to decoy state quantum key distribution systems. The eavesdropper may get some information ab out the secret key without causing any alarms. Besides, the lower bound of detector efficiency mismatch to run our modified faked states attack successfully with perfect quantum nondemolition measurement ability is also given out, which provides the producers of quantum key distribution systems with a reference and can be treated as the approximate secure bound of detector efficiency mismatch in decoy state quantum key distribution systems.