<p>In this paper, we propose a lightweight quantum key distribution (QKD) protocol for two participants within a unidirectional quantum channel environment that inherently prevents Trojan horse attacks. Our protocol features a novel chain method for encoding and decoding single-photon sequences, thus addressing the common limitations of the traditional QKD protocol, which treats photons independently. A notable advantage of our approach is the simplification achieved by requiring only the disclosure of the first photon basis. Furthermore, our method significantly improves the detection rate of measure-resend attacks. When a single photon of a sequence of photons is attacked by an eavesdropper, the detection rate can reach nearly 16.67% if half of them are decoy photons, offering a 3% enhancement compared to protocols without the chain method. In cases where the entire sequence is attacked, checking just twelves photons can achieve a detection rate of 99%, which is five photons fewer than that required by traditional protocols without the chain method. In addition, a privacy amplification method is introduced for the QKD protocol by sharing a hash function, to maintain high efficiency while enhancing security, as a practical solution for quantum communication.</p>