Bioretention, a management strategy employed to address non-point source pollution in urban settings, has emerged as a prevalent method for the treatment of pollutants in stormwater runoff. However, there is a paucity of research on the ammonium nitrogen transport process in the gas-liquid-solid three-phase of bio-retention. The present study investigates the distribution of ammonium nitrogen in a gas-liquid-solid three-phase system. To this end, the study adds various pro-portions (0%, 2%, 4%, 6%, 8%, and 10% by volume) of soil fillers (newspaper, wood chips, and humus leaves) to sandy soil, plants ryegrass, and constructs a laboratory phytoretention system to artificially simulate rainfall. The distribution of ammonium nitrogen in the gas-liquid-solid three-phase system of the sandy soil, with different proportions of soil fillers, was investigated. The study focused on the distribution of ammonium nitrogen in the gas-liquid-solid three-phase system of the phytoretention system. The present study explores the alterations in the system of ammonium nitrogen transport within the gas-liquid-solid three-phase configuration. The volatile accumulation of ammonia in the sandy soil group accounted for 4.67% of the ammonium nitrogen input. The ammonia nitrogen con-tent of the sandy soil group that was discharged from the system with rainwater accounted for 0.24% of the total ammonia nitrogen input. The percentage of soil ammonia nitrogen accumulation to ammonia nitrogen input for the sandy soil group was 51.84%.