Abstract: This paper concerns analytical modeling of water-turbine-electricity (WTE) coupling in the hydroelectric system with a long diversion tunnel, surge tank and penstock at a hydroelectric station. The flow transients in the tunnel are described using the classical rigidity water hammer theory, and the fluid dynamics in the surge tank considered as a transient accumulator, is modeled with the surge equations and dynamically connected with the tunnel outlet and penstock inlet. We have applied the fluid-structure-interaction (FSI) theory to develop a four-equation model for simulation of the transient flows in the penstock, and adopted the FSI junction coupling mode for dynamic connection of the penstock with the turbine wicket gates. A first-order equation of electricity-generating units and a governor equation of their control system are used to fully couple the hydro turbine system. Hence, we have constructed a WTE coupling model that is solved by the inverse Laplace transform of its state equations, and formulated a holistic analysis method of the transients in WTE coupling system. The simulation results show that this approach makes it possible to model various types of transients in WTE coupling system and the modeling has good responses to power load rejection and predicts the system dynamical parameters satisfactorily and efficiently.