Leveraging seawater toilet flushing system in Hong Kong, China, a Seawater-based Urine Phosphorus Recovery (SUPR) process that integrates ureolysis and phosphorus (P) recovery was proposed in our earlier work. In this study, a thermodynamic model was applied to evaluate the effects of ureolysis and the seawater-to-urine mixing ratio (S/U ratio) on P precipitation in the SUPR system. The results suggested that effective P recovery was thermodynamically feasible across a wide range of S/U ratios, with elevated pH levels resulting from ureolysis being critical for P precipitation. Furthermore, a SUPR reactor was developed to validate this process. When the hydraulic retention time (HRT) exceeded 3 h and the S/U ratio was lower than 3:1, more than 98% of P could be recovered without urine storage, chemical dosage, or external mixing. Further decrease in the HRT and increase in S/U ratio caused flushing out of fine precipitates, resulting in a relatively low P recovery efficiency. However, this could be advantageous when downstream urine nitrification is implemented, as dilution of urine can alleviate the inhibitory effects of free ammonia and free nitrous acid, as well as overcome the P limitation problem, thus facilitating urine nitrification. Consequently, there is a trade-off between optimizing P recovery and nitrification efficiencies.