Abstract: In principle, quantum key distribution (QKD) can be used to make unconditionally secure private communication. However, the security of existing real system for QKD needs to be carefully examined. Actually, the existing experiments based on weak coherent states are insecure under photon-number-splitting attack. Fortunately, the decoy-state method and the entanglement-distribution method can be used to realize the unconditionally secure QKD based on real-life system with existing technology.