Amid growing global demand for lithium resources, the efficient recovery of lepidolite has gained significant attention. However, fine-grained lepidolite particles often remain underutilized in conventional beneficiation processes due to their poor floatability and high surface energy. Therefore, it is essential to develop effective methods for separating fine lepidolite from associated gangue minerals. In this study, we systematically investigated the flotation behavior of fine lepidolite (–20 μm) and quartz (–40 μm) in the presence of depressants. The separation mechanism was elucidated through micro-flotation tests, zeta potential analysis, adsorption capacity measurements, wettability studies, and X-ray photoelectron spectroscopy (XPS). The results demonstrate that sodium metaphosphate (SM) acts as an effective depressant for lepidolite in a tetradecyltrimethylammonium chloride (TTAC) system. Under optimized conditions (pH = 7, [TTAC] = 30 mg/L, [SM] = 100 mg/L), a lepidolite concentrate with a Li₂O grade of 2.93% and a recovery of 68.40% was achieved. Mechanism studies revealed that SM competes with TTAC for adsorption sites on lepidolite surfaces, while enhancing TTAC adsorption on quartz. Furthermore, SM exhibits a limited effect on lepidolite, thereby increasing the separation efficiency between quartz and lepidolite. These findings provide theoretical insights into the efficient separation of fine lepidolite from quartz, offering a potential strategy for improving lithium resource utilization.