The　effect　of　the　combination　of　fly　ash　and　silica　fume　on　the　water　resistance　of　Magnesium-Potassium　Phosphate　Cement　(MKPC)　was　investigated,　and　the　improvement　mechanism　was　discussed　based　on　the　micro-analysis　of　XRD,　FSEM　and　pore　structure.　The　results　indicate　that,　the　physical　effect　of　the　combination　of　fly　ash　and　silica　fume　on　MKPC　is　dominated　compared　with　the　chemical　effect　because　of　the　optimization　of　pore　structure.　The　combination　of　fly　ash　and　silica　fume　leads　to　higher　density　and　later-age　compressive　strength　of　MKPCs　compared　to　those　without　fly　ash　and　silica　fume　under　air　curing　and　water　curing,　respectively.　The　strength　retention　ratios　(namely　water　resistance)　of　MKPCs　with　the　combination　of　fly　ash　and　silica　fume　at　56　days　are　found　to　be　significantly　higher　than　those　without　fly　ash　and　silica　fume.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.