实际量子密钥分发系统使用的单光子源主要是由弱激光脉冲经衰减得到。它不是理想单光子源而是服从泊松分布的准单光子源。每个非空光脉冲中包含多光子的概率不为零，强大的窃听者可利用此获得一些关于最终密钥的信息。因此，有必要研究实际QKD系统的安全性。采用对多光子进行分束窃听、单光子最佳攻击相结合的方案，用Shannon信息理论分析了基于弱相干光的实际QKD系统的安全性。研究结果表明实际QKD系统对于分束窃听和最佳攻击是安全的，并给出合法通信双方在该攻击方案下所容忍的误码率上限。

The single-photon sources used in most of the practical QKD systems are obtained through attenuating the weak laser pulse. It is not a perfect single-photon source,but is a quasi-single-photon source which follows the Poissonian distribution. The probability of every nonempty light pulse containing multiphoton is nonzero,the powerful eavesdropper(Eve)can take advantage of this to obtain some information about the final key. So it is necessary to study the security of the practical QKD system. Supposing,Eve launches photon-number-splitting(PNS)attack on multiphoton signals,and optimal attack(OA)on single-photon signals,adopting Shannon's information theory to analyze the security of the practical QKD system based on weak coherent light. The result shows that the practical QKD system is secure against the attacks of PNS and OA,and the upper bound of the error rate that the legitimate users(Alice and Bob)can tolerate is also derived from are study.