Abrupt　salt　intrusion　in　municipal　wastewater　treatment　plants　has　triggered　serious　disturbances　and　deteriorated　the　biological　treatment　process　from　steady　state　into　unsteady　state.　The　effects　of　sodium　(Na+)　concentration　on　the　structure　and　properties　of　activated　sludge　during　the　shock　period　were　investigated　in　this　paper.　A　floc　disintegration　characterized　as　the　decreased　floc　size　and　increased　porosity　in　the　floc　matrix　was　found　when　excess　Na+　was　added　to　the　bioreactor.　Increased　Na+　resulted　in　a　slight　decrease　in　the　normalized　capillary　suction　time.　About　53-57%　of　the　total　extracellular　polymeric　substances　(EPS)　derived　from　activated　sludge　was　protein,　at　Na+　concentrations　＜150.74　mmol/L,　and　this　value　even　reached　68%　at　286.04　mmol/L　Na+.　Over　50%　of　Na+　in　EPS　was　found　in　the　loosely　bound　EPS　(LB-EPS).　The　Na+　sorbed　in　LB-EPS　followed　Langmuir　model　with　a　maxium　sorption　capacity　of　2.086　mmol/g　VSS.　With　the　increase　of　Na+　concentration　in　LB-EPS,　the　Mg2+　concentration　in　LB-EPS　decreased　until　reaching　equilibrium　while　there　was　no　significant　change　of　Ca2+　concentration.　A　concept　model　is　proposed　for　the　first　time　targeting　to　improve　the　understanding　of　sodium　ion　effects　on　activated　sludge.