Carbonaceous phyllite formations are characterized by low strength, developed joints and fractures, and water-induced softening, which are prone to frequent collapses, cavings, and severe wellbore enlargement during drilling. Based on an analysis of the causes of rock water sensitivity, this study investigated an environmentally friendly, low-solid, anti-collapse drilling fluid system incorporating ultra-fine materials composite polyvinyl alcohol (PVA), and explored the synergistic plugging mechanism between ultra-fine materials and PVA. The following conclusions were drawn: The high total clay mineral content of 55.20% and the presence of micron-scale pores are the main reasons for the wellbore instability during drilling in carbonaceous argillaceous phyllite in the mining area. Compared to the base mud, the PVA drilling fluid formed with 1% cold-soluble PVA#2488 reduced the filter cake thickness by 62.27%, and increased the filter cake hardness and shale rolling recovery rate by 110.49% and 7.05%, respectively. The ultra-fine particle composite PVA drilling fluid system constructed with 3000-mesh CaCO? reduced the medium-pressure fluid loss to 1.75 mL and increased the rolling recovery rate to 93.6%. The rapid plugging of micron-scale pores in phyllite by ultra-fine CaCO?, along with the filling and bonding enhancement of the network structure formed by the mutual adsorption of PVA and bentonite, synergistically improved the inhibition and anti-collapse effectiveness of the system.