<p>Quantum secure direct communication (QSDC) can transmit the secret message directly. Single-photon-based QSDC protocol requires the active modulation of the quantum state of the source, potentially introducing various side-channel vulnerabilities. In this paper, we propose a passive-state preparation QSDC protocol. By utilizing a passive-state preparation method, we remove the need for active modulation, thus eliminating the side-channel risks associated with the source encoder. To evaluate the performance of the protocol, we develop a system model and improve the decoy state method. Through parameter optimization, we identify the optimal secrecy message capacity for various communication distances and compare it with active modulation QSDC. At a communication distance of 10 (15) km, the secrecy message capacity of passive-state preparation QSDC reaches 3.894 × 10⁻⁴ (3.715 × 10⁻⁵) bit/pulse, achieving 95.3% (91.5%) of the active case’s performance. Meanwhile, we consider the resource consumption and optimize the secrecy message transmission rate of passive-state preparation QSDC. Using a coherent light source with a frequency of 10⁶ Hz, at communication distances of 5, 10, and 15 km, the secrecy message transmission rates for passive-state preparation QSDC reach 2.370 × 10², 4.218 × 10, and 2.504 bit/s, respectively.</p>