Aluminum dross is a kind of hazardous wastes produced from aluminum electrolysis, processing and regeneration industry. Direct stacking is easy to cause serious environmental pollution and waste of resource. Nevertheless, aluminum dross containing alumina more than 70% can be used to produce poly aluminum chloride. In order to realize the utilization of aluminum dross and solve the pollution problem of coal washing waste water, the coal washing waste water was treated with aluminum dross based poly aluminum chloride (AD-PAC). And the effects of AD-PAC addition amount, temperature, pH, mixing speed for stirring, mixing time for stirring, reaction speed for stirring, reaction time and sedimentation time for stirring on removing the turbidity of coal washing waste water were systematically investigated. The binding mechanism between AD-PAC and particle in coal washing wastewater was discussed by the change of flocculating constituent particle size. And the flocculating mechanism of AD-PAC for coal washing waste water was expounded. The results show that the turbidity removal ratios reach maximum values 99.08%, and the turbidity of coal washing waste water decreases from 54.63 NTU to 0.50 NTU with the conditions of AD-PAC addition 100 mg/L, temperature 40 ℃, pH=10, mixing speed for stirring 300 r/min,mixing time for stirring 30 s, reaction speed for stirring 100 r/min, reaction time for stirring 45 min and sedimentation time 20 min. The average particle sizes of flocs increase from 59.67 μm to 145.90 μm. The AD-PAC make the particles unstable by electric neutralization/adsorption and net capture respectively, and make the particle transmit to large particle flocculates to purify coal washing waste water. The research in this paper will provide a strong support for the application of AD-PAC in wastewater treatment industry.