For first time, an experimental and computational study has been conducted to investigate the properties of magnesium phosphate cement (MPC) mortar and its main hydration product struvite-K crystal that would account for the mechanical properties of these materials. Two types of MPC mortars, magnesium sodium phosphate cement (MNPC) and magnesium potassium phosphate cement (MKPC), were synthesized with magnesium to phosphate (M/P) ratio from 4 to 18 and water to solid ratio (w/s) from 0.16 to 0.2. Subsequently, the setting behavior, compressive strength and water resistance of MNPC and MKPC were investigated to study the influence of M/P and w/s ratios. An increase of M/P ratio can slow down the setting rate for both MPC mortar, and MNPC shows faster setting performance, as compared with MKPC. With increasing M/P ratio, the mechanical properties of MKPC mortar are gradually weakened, while the strength for MNPC mortar is first improved and then degrades, with the optimum value at M/P ratio of 9.5. Additionally, the saturated MPC mortars, immersed in water solution, are significantly weakened, showing poor water resistance. Micro-characterizations including X-ray diffraction and scanning electron microscopy of the MPC mortar reveal that MKPC mortars have good crystallization of struvite-K, with the morphology of plate packing at early hydration stage and the MNP hydrates show amorphous phase. Furthermore, molecular dynamics was conducted to study the structure, dynamics and mechanical properties of the main hydrated phase in MKPC cement, struvite-K crystal. The hydrolytic weakening effect of water molecules and failure mechanism have been unraveled that the H-bonds and K-Os connections lose the chemical stability due to water attacking and tensile loading, which explains the severe failure of MPC mortar observed in the water resistant experiments.