In this study, the simultaneous recovery of phosphorus (P) and potassium (K) from synthetic urine by crystallization of magnesium potassium phosphate hexahydrate (MPP) was studied. A series of conditional experiments were performed to examine the effects of phosphate and magnesium dosages, solution pH, and the presence of Na+, NH4+, and Ca2+ ions on the MPP crystallization. The results revealed that the dosages of phosphate and magnesium had a marked effect on the MPP crystallization. Separate excess of magnesium was not beneficial for the formation of MPP, while the simultaneous excess of magnesium and phosphate could significantly improve the efficiency of MPP precipitation. The optimum pH for MPP crystallization was determined to be 11. At this pH, the presence of Na+, NH4+, and Ca2+ in the aqueous solution markedly influenced MPP crystallization. The effects of NH4+ and Ca2+ on the removal of K were far greater than that of Na+. Moreover, in the crystal seeding technique on MPP crystallization, the K- and phosphate-removal efficiencies were found to increase by about 10%, with an increasing dosage of the preformed MPP from 0 g/L to 30 g/L. Thus, a continuous-flow MPP precipitation process is recommended for the treatment of synthetic urine and real urine as the results demonstrated that the phosphate and K could be efficiently removed from both the urine types.