This paper studies a measurement-device-independent quantum key distribution protocol based on a multiple crystal heralded source and pulse position modulation to improve its performance. The performances of the protocols with or without pulse position modulation are compared under the multiple crystal heralded source. The simulation results show that the application of pulse position modulation can further improve the key generation rate and increase secure transmission distance of the protocol. Moreover, as the time slot increases, the key generation rate and the secure transmission distance gradually rise. Furthermore, we analyze the relationship between the secure transmission distance and the key generation rate under different detection efficiencies of the detector. The results show that the higher detection efficiency of the detector leads to the greater key generation rate and the longer secure transmission distance.