The　solubility,　supersaturation　and　metastable　zone　width　of　magnesium　ammonium　phosphate　(MAP)　in　aqueous　solution　were　measured　at　4,　20,　30　and　40℃　by　laser　device　using　the　pH　value　as　the　control　parameter.　While　the　changing　of　metastable　zone,　the　induction　period　and　the　crystal　reaction　rate　of　MAP　crystallization　were　investigated　simultaneously.　The　results　showed　that　to　raise　pH　value　could　decline　the　solubility　and　supersaturation　of　MAP,　but　enlarged　the　metastable　zone　width.　However,　the　solubility　and　supersaturation　of　MAP　showed　an　upward　trend　with　the　increasing　of　temperature,　as　well　as　the　crystallization　metastable　zone　width　would　be　narrowed　with　the　increasing　of　temperature　or　MAP　concentration.　It　was　also　observed　that　the　MAP　crystallization　induction　period　was　gradually　shortened　and　the　crystallization　reaction　rate　was　significantly　improved　with　the　increasing　of　pH　value.　Generally,　in　the　crystallization　phosphorus　removal　process,　the　crystallization　reaction　of　MAP　could　be　promoted　with　the　optimizing　of　crystallization　process　by　pH　controlling.　©,　2015,　Science　Press.　All　right　reserved.