In this paper, the effect of metakaolin on the performance of magnesium potassium phosphate cement (MKPC) with a high water-to-solid (w/s) ratio of 0.375 was studied. MKPC was prepared by substituting metakaolin for varying amounts of magnesia. The results showed that the increase of metakaolin led to a decrease in the fluidity of the fresh MKPC slurry. Additionally, the presence of metakaolin reduced the maximum temperature of the second peak in the MKPC hydration reaction and delayed its appearance. This is because of the amount of magnesia as an effective alkaline component decreased. Metakaolin improved the compressive strength of MKPC by reducing the porosity and forming new hydration products. Moreover, it was found that the compressive strength of MKPC with 30% metakaolin exhibited the highest at 20.36 MPa at 28 days and showed a downward trend when metakaolin exceeded 30%. Metakaolin owns the pozzolanic effect as well as the filling effect in the MKPC system of a high w/s ratio, which is the reason for the improvement in compressive strength and reduction of chloride diffusion. This work provides some fundamental information for understanding the high w/s ratio MKPC modified by metakaolin, which helps to promote its application in the field of anticorrosion coatings and waste stabilization/solidification binders.