Abstract: The alternating process of open-channel and pressurized flows in a water distribution system is a major cause of system damage. Accurate simulation of the transient process of mixed openpressurized flow in water distribution systems is a technical problem that urgently needs to be solved.However, the commonly used Preissmann narrow slot method cannot simulate the transient process with negative pressure flow, greatly limiting its applicability to these systems. This paper develops a numerical model for unsteady open-channel flow, pressurized transient flow with negative pressure, and openpressurized mixed flow with negative pressure, through coupling the dichotomy pressure method and the Preissmann negative slot idea. We develop a method for distinguishing negative pressure flows from open-channel flows based on the CFL constraints, and apply the model to complicated flow cases-openpressurized flow alternation and simultaneous action of water hammer during open channel filling, water hammer in the pressurized pipeline, and valve closing during the filling. The applications show that our new model breaks through the limitation of the Preissmann method and the difficulty in distinguishing open-channel flows and negative pressure flows by the differential pressure method, and achieves accurate simulations of open-pressurized alternation transients without gas-liquid two-phase interaction.