To further improve the removal of the dissolved organic matter (DOM) in secondary effluent during the laboratoryscale soil aquifer treatment (SAT), flyash (FA) was added to the traditional soil columns as an additive in different proportions and filling patterns. Removal and transformation of the trihalomethanes (THMs) precursors in secondary effluent in the FAadditive SAT system was investigated by the analysis of ultraviolet absorbance at 254 nm (UV²⁵⁴), dissolved organic carbon (DOC), specific ultraviolet light absorbance (SUVA) and excitationemission matrix (EEM) fluorescence spectroscopy. The FAadditive enhanced the reduction of DOC, UV²⁵⁴, SUVA and THMFP efficiently. The increasing of FA dosage would lead to a high removal efficiency of DOM during SAT operation, but the biodegradation of the columns was negatively affected. Experimental results demonstrated that the combination of FA adsorption and biodegradation in soil could achieve a better removal of DOM. Fluorescence intensity showed that the FAadditive could effectively remove aromatic acid proteins, humiclike and fulviclike materials (THMs precursors) in secondary effluent.