The new energy access ratio has rapidly increased in recent years, leading to increasingly prominent frequency stability issues in regional power grids. Therefore, this article studies the relationship between access ratio and frequency stability and proposes an improved control strategy for hydropower and thermal power-dominated frequency regulation. The analysis shows that as the access ratio increases, the equivalent inertia of the regional power grid decreases, and the equivalent power-frequency regulation ability decreases, resulting in a decrease in the static, dynamic, and transient frequency stability of the hydroelectric-dominated regional power grid; This leads to a decrease in the static and transient frequency stability of the power grid in the thermal power-dominated area, while the dynamic frequency stability remains unchanged. By reducing the adjustment coefficient and adding differential links to the frequency feedback channel, the overall frequency stability of hydropower-led frequency regulation is improved. Adding a proportional differential link to the frequency feedback channel improves the overall frequency stability of thermal power-led frequency modulation. The simulation results indicate that the analysis of the influencing factors is reasonable and that the improved control strategy is effective. This study has significant reference value for improving the frequency stability of large-scale new energy access systems.