<p>Fully passive quantum key distribution (QKD) avoids active modulation of the source and enhances robustness against side-channel attacks. However, existing fully passive QKD systems suffer from low resource efficiency and dependence on precisely calibrated reference frames, limiting their practical applicability. We present a fully passive reference-frame-independent (RFI) QKD protocol that eliminates side channels caused by source-side modulators and fully utilizes the quantum states generated by passive sources to improve efficiency. Our protocol requires calibration of only one axis of the reference frame for secure key transmission, significantly reducing implementation complexity. We developed a system model and conducted parameter optimization, which resulted in a secure key transmission rate improvement of approximately 20% compared to the original fully passive QKD under ideal scenarios. In addition, we analyzed the protocol’s performance for finite-length keys. Our fully passive RFI-QKD protocol contributes to the development of practical fully passive QKD systems.</p>