To enhance the compatibility, dispersibility, and flame retardant effect of antimony trioxide (Sb₂O₃) in polypropylene (PP), Sb₂O₃ is often modified by surface modifiers in industry. This study employed the aluminate coupling agent DL-411 for the wet surface modification of Sb₂O₃. The experiments investigated the effects of slurry pH, DL-411 dosage, modification time, and modification temperature on the water contact angle of modified Sb₂O₃ (Al-Sb₂O₃) and the viscosity of Al-Sb₂O₃/liquid paraffin suspension. Fourier transform infrared spectrometer (FTIR) characterization was conducted on Sb₂O₃ before and after modification with aluminate coupling agent DL-411. The optimized conditions for the modification were determined as follows: pH value9, DL-411 mass fraction of 1.5%, modification time of 5 min, and modification temperature of 30 ℃. The obtained Al-Sb₂O₃, compounded with decabromodiphenyl ethane (DBDPE) as a flame retardant, was melt-blended with PP at a 25% mass fraction. The composite flame retardant material was tested for flame retardancy, mechanical properties, and melt flow rate. A thermogravimetric analyzer (TGA) and a scanning electron microscope (SEM) were used to characterize its thermal stability and microstructure. The flame retardancy and mechanical properties of PP/Al-SbBr composite flame retardant material were significantly improved compared to the same loading of PP/SbBr composite flame retardant material. The findings indicate that adjusting the pH value to an appropriate level allows the aluminate coupling agent DL-411 to effectively enhance the flame retardancy of Sb₂O₃-modified composites with reduced degradation of mechanical properties, even in an environment close to room temperature and with a relatively short modification time.