Aiming at the disadvantage of the active decoy-state scheme to introduce extra information, we investigate the passive decoy-state measurement-device-independent quantum key distribution (MDI-QKD) protocol based on heralded pair coherent states and pulse-position modulation by means of the rotation-invariant photonic state. The performance comparison among the traditional MDI-QKD protocol, the passive decoy-state MDI-QKD protocol based on rotation-invariant photonic states, and the passive decoy-state MDI-QKD protocol based on rotation-invariant photonic states with different frame lengths is conducted. The simulation results show that the key generation rate and the secure transmission distance can be improved if rotation-invariant photonic states and the pulse-position modulation are introduced. Moreover, with the increase of frame length, the performance of the protocol is also improved. Therefore, in the absence of intensity modulation, this protocol can avoid the influence of light side channels and can be used to increase the key generation rate.