Unsteady transient phenomena in flow over impulsively started circular cylinder, such as the generation of separation, burst of separation bubble, vortex shedding, etc., are studied from Lagrangian viewpoint. The transient flow is solved numerically by using characteristic-based split scheme with dual time stepping. Then Lagrangian coherent structures (LCSs) are extracted to study the transport and mixing of these transient phenomena. Results show that the variation of drag is closely related to the evolutions of separation bubbles and vortex shedding. The evolutions of the symmetric bubbles in streamwise induce high pressure distribution at rear of cylinder and result in drag reduction of the circular cylinder. As separation bubbles become asymmetric, the transport between separation bubbles and main flow is enhanced and thus can reduce the separation region and suppress flow separation as well. The results also show that the shedding vortices are induced by the transpor between separation bubble and main flow. Compared with streamline patterns, LCSs have huge advantages in describing the dynamic features of the unsteady phenomena.