Abstract: This paper takes the ultrafine coal slime (D₅₀=9.556 μm) from Jining No. 2 Coal Mine as the research object, systematically studies the effects of four types of filter aids (sodium dodecyl benzene sulfonate (SDBS), aluminum sulfate octadecahydrate (ASO), polyaluminum chloride (PAC), and composite filter aid (CFA)) and their dosages on the filtration and dewatering efficiency of coal slime, and analyzes the influence of the pH value of the coal slime slurry on the dewatering effect. The results show that CFA can significantly improve the dewatering efficiency of coal slime, reducing the cake formation time from 150 minutes to 25 minutes at a dosage of 2%. The primary reason for promoting rapid dewatering of coal slurry is the synergistic neutralization of the surface charge of coal slurry particles by the composite filter aid (Zeta potential increased from −30.9 mV to −5.52 mV) and the moderate enhancement of hydrophobicity (contact angle increased from 24.6°to 33.9°). Notably, although SDBS can reduce the moisture content of the filter cake to 22.28%, it significantly prolongs the dewatering time to 450 minutes due to its promotion of foaming in the coal slurry. Additionally, weakly acidic conditions can further leverage the filtration advantages of CFA. When the pH is 5, the cake formation time for coal slurry with 4% CFA is only 20 minutes. This study provides a preferred filter aid selection scheme and theoretical support for the efficient dewatering of ultrafine coal slurry.