Background <p>This study aims to investigate the prevalence and morphology of middle mesial canal (MMC) in mandibular first molar (M1M) among a Northwestern Chinese population, and to analyze their relationship with anatomical aspects of the mesial root.</p> Methods <p>Cone beam computed tomography (CBCT) was utilized to evaluate 898 M1Ms and assess the incidence and morphology of MMC. The following parameters for M1M with or without MMC were obtained: the vertical distance between the first appearance of MMC and canal orifices (D), the distance between mesiobuccal (MB) and mesiolingual (ML) canals (D1), the buccolingual width(L1) and mesiodistal width (L2) of mesial roots, and the flatness degree(L1/L2) of mesial roots. The results were statistically analyzed.</p> Results <p>The prevalence of MMC was 9.6% when considering the number of teeth and 7.2% when considering individuals. The presence of MMC was not significantly associated with sex (<i>p</i> = 0.993) or age (<i>p</i> = 0.211). Type 1-3-2 emerged as the most prevalent root canal morphology. MMC primarily manifested within 4&#xa0;mm below the canal orifices. In cases where MMC was present, the MB-ML distance was significantly greater (<i>p</i> = 0.017). Conversely, no significant correlation was found between the presence of MMC and the length, width, or flatness degree of the mesial roots.</p> Conclusions <p>The morphology of MMCs is complex, and most of them exhibit confluent canals. In instances where MMCs are present, the MB-ML distance is significantly larger. For effective detection of MMC, a thorough examination of the area within 4&#xa0;mm beneath the canal orifice is recommended.</p>