It　is　important　to　clarify　the　mechanism　of　the　cake　layer　formation　in　the　early　stage　of　filtration　(initial　stage　and　pseudo-steady　stage).　In　this　paper,　a　series　of　short-term　dead-end　microfiltration　of　sludge　suspensions　domesticated　at　three　different　ratios　of　carbon　to　nitrogen　(C/Ns)　(63:10,　100:5,　200:5)　were　conducted　in　constant　pressure　mode.　Flux　decline　behaviors　were　analyzed　and　the　structure　of　cake　layer　was　characterized　by　3D　optical　digital　microscope　(3D-ODM)　to　explore　the　mechanism　of　the　cake　layer　formation.　Results　showed　that　the　sludge　floc　was　the　key　substance　in　the　initial　stage　of　the　cake　layer　formation　while　colloid　matters　dominated　later.　Based　on　the　inhomogeneous　structure　of　the　formed　cake,　a　dynamic　traversing　mechanism　was　proposed　to　show　the　fact　that　colloid　matters　traveled　through　the　hole　formed　by　sludge　flocs　and　accumulated　on　the　membrane　surface.　Meanwhile,　a　piecewise　function　about　the　relationship　of　accumulated　filtrated　volume　(v)　and　filtration　time　(t)　was　established　to　reveal　this　mechanism.　Moreover,　the　model　predictions　showed　good　agreement　with　experimental　data　and　its　applications　were　also　approved　by　other　activated　sludge　suspensions,　kaolin　suspensions　and　real　wastewater.