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Conventional bioretention filters lack satisfactory performance in nitrogen removal. In this study, we used a mixture of cultivated soil and river sand as the bioretention filter to remove nitrogen pollutants from simulated rainwater runoff. To improve its permeability and nitrogen removal performance, both activated carbon and ceramsite were used as additives. The nitrogen removal processes and its mass accumulation in the modified bioretention filters were studied. The contribution of adsorption and biotransformation processes, together with the effects of percolate rate on nitrogen removal performance was explored. The results showed that an activated carbon layer in the bioretention filters could obviously improve nitrogen removal efficiencies, but its location made no significant difference in nitrogen removal performance. Bioretention filters modified with 20% of ceramsite could achieve the optimal percolate rate and nitrogen removal efficiencies. At given conditions, the average removal efficiencies of ammonium nitrogen (NH3–N), nitrate-nitrogen (NO3–N), and total nitrogen (TN) by the modified bioretention filter reached 80.27%, 41.48%, and 59.45%, respectively. During the leaching processes, organic nitrogen originated in the filter materials can be mineralised into NH3–N, then be denitrified and completely removed in the anaerobic environment under flooding conditions. Biotransformation in the modified bioretention filters caused a reduction of NH3–N removal efficiency by 15.41% and an increase of NO3–N removal efficiency by 31.03%. The modified bioretention filter can withstand a long-term operation. Compared with NO3–N and TN, the pollutant of NH3–N in rainwater runoff is not easy to form a mass accumulation in the modified bioretention filter. Highlights The modified bioretention filter showed high percolation rate and nitrogen removal. Hydraulic residence time is a critical design parameter to achieve nitrogen removal. NH3–N is not easy to form a mass accumulation in the filler media as NO3–N. Biodegradation increased NO3–N removal efficiency by 31.03% at given conditions. [ABSTRACT FROM AUTHOR]