<p>Road tunnels significantly enhance transportation efficiency. However, the recurring threat of tunnel fires necessitates advanced safety measures; the integration of water mist systems with longitudinal ventilation provides a reliable method for fire suppression and smoke exhaust, which is vital for protecting both tunnel structures and human life. This study investigates the impact of nozzle arrangements, specifically single-, double-, and triple-row configurations on smoke back-layering length under longitudinal ventilation. To ensure comparability, all arrangements were evaluated using a constant total flow rate. The study characterizes the variation patterns of smoke back-layering length relative to nozzle arrangement and flow rate. Through non-dimensional analysis, a prediction formula was developed to quantify the relationship between flow rate and back-layering length across different nozzle configurations. The results demonstrate that the triple-row arrangement provides the superior control effect. This is because, at a constant flow rate, distributing water through a higher number of nozzles increases the coverage area of the smoke layer, thereby enhancing the cooling effect and effectively reducing the back-layering length. Specifically, the triple-row arrangement can reduce the back-layering length by up to 25% compared to the single-row configuration.</p>