量子密钥分配过程中制备诱骗态信号易引入一些边信息(频率、脉冲宽度等)，窃听者可利用这些信息来分辨信号态和诱骗态。因此，提出了基于参量下转换光源和被动诱骗态方案的测量设备无关量子密钥分配协议，分析了其密钥生成率、单光子计数率以及单光子误码率与安全传输距离的关系。仿真结果表明，基于参量下转换光源的被动测量设备无关量子密钥分配协议的密钥安全传输距离达到285 km，远高于基于改造后可输出两路相关信号的弱相干光源的被动测量设备无关量子密钥分配协议，十分接近基于主动诱骗态的测量设备无关量子密钥分配协议，且克服了主动诱骗态方案可能引入边信息的缺点。

The preparation of decoy state in the quantum key distribution process is easy to introduce some extra information (frequency, pulse width, etc.), and the eavesdropper can use these information to distinguish signal state from decoy state. Therefore, this article proposes passive measurement device independent quantum key distribution (MDI-QKD) based on parametric down conversion source (PDCS) and decoy state plan, and analyzes the key generation rate, single photon counting rate, bit error rate and the secure key transmission distance. The simulation results show that the maximum secure distance of passive MDI-QKD based on PDCS is about 285 km, which is much longer than MDI-QKD based on modified weak coherent source and is close to active MDI-QKD, and overcomes the imperfection of active decoy state plan that may introduce the extra information.