Unsteady flow separation is a common phenomenon,especially for flow with unsteady control techniques.Its well known that the flow separation is a Lagrangian phenomenon,and its dynamic properties,such as transport of fluid,is essential to the separated flow downstream and the aerodynamic performance of structure.These dynamic properties can only be revealed from Lagrangian viewpoint.However,there still has no efficient description for unsteady separation due to its complexity,and Lagrangian description of unsteady flow separation still need to be studied further.In this study,the dynamic properties of unsteady flow separation are analyzed from Lagrangian viewpoint.Special attention is paid on the transport and mixing in the unsteady flow separation near the non-slip boundary condition.Then,the dynamic behaviors of several typical unsteady flow separations are studied in detail.The results show that flows in the boundary layer and main flow have different time scale in the transport process.As a result,the fluid near the no-slip wall cannot separate in finite time,and the onset of flow separation begins in the interior of fluid.Therefore,in the boundary layer,the transport of fluid is suppressed due to the non-slip boundary condition.The results also show that the unsteadiness of flow can enhance the fluid transport and mixing between main flow and separated flow,resulting in the reduction of separated area.