To optimize the bioretention mixed fillers with better removal of phosphorus, this paper studies the adsorption characteristics of single filler and modified mixed filler through static adsorption experiments, and adopts the dynamical mini-column experiments to examine the adsorption capacities of the soil and modified mixed fillers. Results show that, in the static adsorption experiments, both water treatment residual (WTR) and fly ash exhibit good adsorption capacity when used as a single filler and modifier. Adsorption capacity increases with increasing WTR and fly ash dosage in the mixed filler. The modified mixed filler with WTR exerts a clear effect in the dynamic adsorption experiment, which is unsaturated when influent phosphorus concentration is 1 mg/L and inflow amount is equivalent to 15 years of precipitation. The adsorption capacity of WTR is 3.5–4.5 times that of other mixed fillers. Fly ash as a modifier shows a poor dynamic adsorption effect and thus must be continuously studied. In this study, WTR is recommended as a bioretention phosphorus removal additive. In engineering applications, the amount of WTR added can be controlled within 5–10% (by mass) according to influent phosphorus concentration.