The　microscopic　characteristics　of　escape　behaviours　from　a　three-dimensional　space　containing　stairs　have　huge　influence　on　the　evacuation　efficiency,　the　formation　of　congestion　and　the　safety　of　pedestrians,　since　if　emergency　occurred,　a　misplaced　step　of　pedestrians　can　result　in　serious　injury　if　it　leads　to　a　fall.　In　this　paper,　a　series　of　human　evacuation　experiments　under　conditions　of　good　and　zero　visibility　are　firstly　performed　in　a　3D　lecture　theatre.　It　is　demonstrated　that　the　swaying　of　trajectories　both　laterally　and　vertically　are　essentially　caused　by　the　human　biomechanical　features.　Observation　of　the　video　recording　and　trajectories　reveals　several　typical　forms　of　behaviour　related　to　route　choice,　evacuation　direction　and　following　behaviour　under　zero　visibility,　which　are　distinctly　different　from　those　in　normally　lit　conditions.　Moreover,　there　exist　two　high　density　areas　for　normally　lit　and　blind　evacuations:　the　exit　area　and　the　transition　region　between　the　stairs　and　the　ground　in　the　aisle,　but　the　congestion　in　the　aisle　is　more　serious　when　visibility　decreases　to　zero.　Based　on　experimental　observation　and　analysis,　we　propose　a　3D　social　force　model　by　introducing　two　lateral　and　vertical　oscillation　forces　to　reproduce　the　realistic　swaying　characteristic　of　pedestrians　escaping　from　a　3D　lecture　theatre.　Finally,　simulations　with　the　same　setup　as　the　real-life　experiments　are　carried　out　using　the　proposed　3D　model　considering　swaying,　and　the　simulated　evacuation　speed　curves,　swaying　trajectories,　density　profiles　as　well　as　pedestrian　position　distributions　all　demonstrate　remarkable　consistence　with　the　experimental　results.