In order to study effect of early seawater curing on microstructure and properties of magnesium potassium phosphate cement(MKPC) paste, the compressive strength and drying shrinkage of MKPC paste specimens under different curing conditions, and the phase composition and microstructure were analyzed.The results show that there is an obviouse effect of early seawater curing on the properiets of MKPC paste. If MKPC paste is hardening and has a certain initial structural strength, the main hydrate MgKPO4·6H2O(MKP) the gradual infiltration of seawater makes the hydration reaction of MKPC paste sustained in sea， and more MgKPO4·6H2O crystals with high crystallization degree and less defect are formed and filled in pores in hardened MKPC paste, and that the structure of harden MKPC paste tends to be dense. For this reason, MKPC paste specimens, were immersed in sea water after 3d natural curing, have higher compressive strength and lower deformation, its 28d and 60d compressive strength are respectively 13% and 5% higher than that of MKPC paste in natural curing condition, and its 60d shrinkage deformation is 12% that of MKPC paste in natural curing condition and 23% than that of MKPC paste in seawater condition after 28d natural curing.