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.