The　growth　of　hydrated　crystals,　NaCl　and　KH2PO4,　was　in-situ　characterized　with　environmental　scanning　electron　microscopy.　The　influence　of　the　aqueous　environment,　including　the　pressure,　temperature,　and　relative　humidity,　φ,　on　its　microstructures　were　studied　with　X-ray　diffraction,　and　environmental　scanning　electron　microscope.　The　results　show　that　the　highly　crystalline　KH2PO4　crystals　grains,　a　square　column　jointed　with　a　square　cone,　can　be　grown　by　controlling　the　relative　humidity　and　the　evaporation-condensation　states　of　water　vapor,　at　a　temperature　of　1~2°C,　a　pressure　of　600~650　Pa,　and　a　relative　humidity　φ　of　91%~95%.　Under　the　optimized　conditions,　at　a　pressure　and　a　temperature,　closed　to　those　in　the　standard　saturated　water　vapor　curve,　the　cubic　structured　KH2PO4　grains　were　grown.