Controlled mud-cap (CMD) drilling is an advanced deepwater drilling technology. When the surface pump is shut down, the CMD system causes unsteady flow that may create control problems. A new mathematical model has been developed to simulate unsteady flow in CMD systems. The model predicts the transient flow, the equilibrium time, and the change in bottomhole pressure during the unsteady flow. The simulation results show that the flow parameters change rapidly during the first 10 min and that unsteady flow leads to fluctuations in bottomhole pressure. In combination with measured data, the transient flow behavior computed by the model can be used for early detection of well kick after shutdown of the mud pump.