Abstract: Daqu serves as the core saccharification and fermentation starter in traditional Chinese Baijiu production. It is characterized by its abundant enzymatic systems, diverse flavor precursors, and complex microbial communities, which together provide the material basis and driving force for fermentation and play an irreplaceable role in shaping the flavor profile of the base liquor. Recent studies have indicated a significant positive correlation between the quality of Daqu and that of the resulting base liquor. However, current research studies have primarily focused on characterizing individual influencing factors, whereas the systematic integration of the key determinants affecting the quality of Daqu and elucidation of their underlying mechanisms remain insufficient. In this review, we comprehensively summarize—from the perspective of brewing microbial ecology—the mechanisms by which raw material properties (including moisture, starch, and protein contents), processing parameters (e.g., particle size), environmental factors (temperature and humidity), storage duration, and modern enhancement strategies (notably, the addition of functional microbial inoculants) influence the quality of Daqu. The aim of this review is to provide theoretical insights and technical support for the improvement and stabilization of Daqu quality for future applications.