Foamed magnesium oxysulfate cement (FMOSC) was prepared by using organophosphonic acid-amino trimethylene phosphonic acid (ATMP) as a modifier, and the effects on dry density, compressive strength, and water resistance of FMOSC were investigated when ATMP was modified alone and when ATMP was modified in combination with potassium dihydrogen phosphate (KDP), respectively. The physical phase composition and microscopic morphology of FMOSC before and after modification were analyzed by a variety of microscopic tests, analyses and characterizations, such as pore structure distribution, XRD, FT-IR, TG-DSC and FESEM. The results show that the compound modification is most effective at m(ATMP)∶m(KDP)=1∶1 with a total dosage of 1.00% (mass fraction, same as below) compared to ATMP single modification, and it has a significant enhancement on the water resistance and pore structure of FMOSC. The addition of the modifier changes the physical phase composition of FMOSC and generates a new strength phase 5Mg(OH)₂·MgSO₄·7H₂O (5·1·7 phase).