Abstract: This paper analyses the transient process of hydro-turbine units with slope ceiling tail tunnels under primary frequency regulation and load regulation in governor power mode through field measurement and numerical simulation, and presents a simulation method based on three-zone modelling of small flow fluctuations in the tail tunnels. We derive the governing equations of the transient process, construct a simulation model of hydro-turbine regulators in governor power mode, and verify it using PLC program governor identification and the measured data. Aimed at the hydro-turbine unit with a slope ceiling tail tunnel, a "slow-fast-slow" three-stage regulation method for improving power mode dynamic quality is developed to reduce power anti-regulation values, increase power regulation speed and ensure operation stability. The strategy and parameters of governor power regulation are optimized using numerical simulations. Results show that through optimization, the power regulation is speeded up obviously, the power anti-regulation value is decreased, and the hydro-turbine transient process of primary frequency regulation and load regulation under the governor power mode is improved significantly.