[Objective] To address the insufficient physical representation and poor adaptability to complex boundaries of the traditional Preissmann slot method in simulating open channel-to-pressurized flow transitions in drainage networks，this study develops a one-dimensional hydrodynamic model using the Godunov-type finite volume method. [Methods] The model employs two sets of governing equations for free-surface and pressurized flows，coupling the Two-Component Pressure Approach （TPA） with an improved Harten-Lax-van Leer with Source term （HLLS） Riemann solver. This establishes a unified simulation framework for open-channel flow，pressurized flow，and mixed open channel-pressurized flow. Accordingly，the model proposes treatment methods for various typical boundaries，including junction nodes，storage tanks，control gates，pump station outlets，and river-lake systems，based on the characteristic line equations and actual pipeline geometric parameters，significantly enhancing its applicability in pipe network systems. The model is validated through classical test cases such as tree-like and looped pipe networks，showing excellent agreement between simulated and expected results. [Result] The study reveals that the widely used SWMM model，which is based on the Preissmann slot method and the “pipe-node” conceptualization approach，produces unrealistic physical phenomena—such as synchronized water level variations at adjacent nodes—when simulating unsteady flow propagation in pipe networks. In contrast，the proposed model accurately captures non-uniform water level rises and transient fluctuation characteristics along pipelines，providing a more realistic representation of flow propagation. [Conclusion] The proposed model is suitable for detailed dynamic simulations of mixed free-surface and pressurized flows in urban drainage networks.