The　initiation　mechanism　of　debris　flow　is　regarded　as　the　key　step　in　understanding　the　debris-flow　processes　of　occurrence,　development　and　damage.　Moreover,　migration,　accumulation　and　blocking　effects　of　fine　particles　in　soil　will　lead　to　soil　failure　and　then　develop　into　debris　flow.　Based　on　this　hypothesis　and　considering　the　three　factors　of　slope　gradient,　rainfall　duration　and　rainfall　intensity,　16　flume　experiments　were　designed　using　the　method　of　orthogonal　design　and　completed　in　a　laboratory.　Particle　composition　changes　in　slope　toe,　volumetric　water　content,　fine　particle　movement　characteristics　and　soil　failure　mechanism　were　analyzed　and　understood　as　follows:　the　soil　has　complex,　random　and　unstable　structures,　which　causes　remarkable　pore　characteristics　of　poor　connectivity,　non-uniformity　and　easy　variation.　The　major　factors　that　influence　fine　particle　migration　are　rainfall　intensity　and　slope.　Rainfall　intensity　dominates　particle　movement,　whereby　high　intensity　rainfall　induces　a　large　number　of　mass　movement　and　sharp　fluctuation,　causing　more　fine　particles　to　accumulate　at　the　steep　slope　toe.　The　slope　toe　plays　an　important　role　in　water　collection　and　fine　particle　accumulation.　Both　fine　particle　migration　and　coarse　particle　movement　appears　similar　fluctuation.　Fine　particle　migration　is　interrupted　in　unconnected　pores,　causing　pore　blockage　and　fine　particle　accumulation,　which　then　leads　to　the　formation　of　a　weak　layer　and　further　soil　failure　or　collapses.　Fine　particle　movement　also　causes　debris　flow　formation　in　two　ways:　movement　on　the　soil　surface　and　migration　inside　the　soil.　The　results　verify　the　hypothesis　that　the　function　of　fine　particle　migration　in　soil　failure　process　is　conducive　for　further　understanding　the　formation　mechanism　of　soil　failure　and　debris　flow　initiation.　©　2017,　Science　Press,　Institute　of　Mountain　Hazards　and　Environment,　CAS　and　Springer-Verlag　Berlin　Heidelberg.