One　novel　lightweight　building　material　was　successfully　manufactured　using　magnesium　oxysulfate　(MOS)　cement　as　binder　and　foam　and/or　rice　husk　as　inert　fillers　in　this　paper.　It　behaves　much　higher　specific　strength　and　lower　environmental　load　than　normal　concretes　prepared　by　Portland　cement,　natural　sand　and　aggregates.　To　improve　the　mechanical　strength　and　water　resistance　of　MOS　matrix,　three　kinds　of　weak　acids　were　incorporated　at　low　levels.　On　the　basis　of　the　modified　MOS　matrix,　foam,　rice　husk,　and　the　mixture　of　foam　and　rice　husk　were　added　at　variable　dosages　as　fillers　to　prepare　MOS　cement　based　lightweight　materials.　Mechanical　strength,　bulk　density　and　compressive　strength/volume　deformation　of　these　materials　were　evaluated　after　variable　wetting-drying　cycles.　At　the　same　time,　phase　composition　and　microstructure　of　typical　samples　were　studied　using　X-Ray　Diffraction　(XRD),　Fourier　Transform　Infrared　Spectrometer　(FTIR),　Optodigital　Microscope　(OM)　and　Scanning　Electron　Microscope　(SEM)　tests.　Experimental　results　indicated　that　the　addition　of　0.5%　citric　acid　by　MgO　weight　can　greatly　enhance　compressive　strength　and　water　resistance　due　to　the　formation　of　finer　needle-like　crystals　of　5　x　1　x　7　phase　instead　of　3　x　1　x　8　phase.　The　higher　incorporation　of　foam　and　rice　husk　decreases　bulk　density　and　mechanical　properties　of　MOS　cement　pastes.　With　a　density　of　as　low　as　1000　kg/m(3),　the　compressive　strength　of　MOS　cement　based　lightweight　materials　can　reach　higher　than　12　MPa.　The　wetting-drying　treatment　leads　to　a　negative　effect　on　both　compressive　strength　and　volume　stability　of　these　materials,　due　to　the　transformation　of　unreacted　MgO　into　Mg(OH)(2)　crystals　upon　water　immersion.　On　the　other　hand,　the　coupling　effect　of　water-swelling　and　drying-shrinkage　induces　the　formation　of　micro　cracks　in　specimens.　A　proper　addition　of　foam　bubbles　and　rice　husk　presents　the　better　resistance　to　wetting-drying　cycles.　Therefore,　the　MOS　cement　based　lightweight　material　could　be　widely　applied　to　inner　partition　walls　and　structural　component,　and　its　long-term　service　behavior　should　be　further　assessed.