Although the reactivity of fly ash (FA) in magnesium potassium phosphate cement (MKPC) systems is still unclear, it has been generally treated as an inert filler in the mix designs of FA/MKPC-based materials. In this study, FA was incorporated in MKPC mortars through two different design methods, where FA was either treated as inert or reactive. Fresh performance, compressive strength and micro-characteristics of the FA/MKPC mortars were systematically evaluated. The experimental results show that the design method where FA was treated as reactive can lead to better fresh performance, better optimized microstructure and higher mechanical strengths. The SEM analysis indicates some reaction of the FA particles within the MKPC mortars; however, the detailed compositions of the formed secondary reaction products could not be directly determined by either XRD or TGA analysis. Furthermore, this study underlines the importance of two ratios, R-Magnesia:FA and RKDP:(Magnesia + FA), in optimum designs of FA/MKPC-based materials.