Fly ash modification by polydimethydiallylammonium chloride (PDMDAAC) in laboratory scale was explored in this work and the adsorptionperformance of modified fly ash and its application in dyeing wastewater treatment were also studied.The key factors (concentration and temperature) for PDMDAAC to affect the adsorption properties of fly ash (FA) were revealed using theorthogonal test with four factors. The results indicated that the adsorption magnitude of fly ash to PDMDAAC increased due to its favorablespecific surface causing the change of the surface charge nature. Hence, adsorption performance of modified fly ash on organic molecules and itsion exchange capacity are strengthened. The maximum color removal efficiency was obtained as 88.2% by modified fly ash with 2.0 g/100mLdosage in dyeing wastewater, which is much higher than 12.5% color removal efficiency by raw fly ash with the same dosage. And, the usedmodified fly ash could be used for cement production as additive agent. The intensity of cement produced with 15% the modified fly ash in weightreached the Chinese Cement Standard (GB/T17671-1999), blazing a promising novel way in fly ash utilization.