Under the context where the dual-carbon goals fuel the transformation of energy structure and large-scale hydro-photovoltaic complementary bases in Southwest China are emerging as the core of inter-regional energy transmission, the large-scale integration of renewable energy reduces the synchronous support capability of the sending-end power grid. Traditional thermal power units respond slowly in frequency regulation, and photovoltaic generation can only provide short-term support due to energy constraints, leading to frequency regulation power deficits and instability during cross-regional transmission. To address these issues, this paper develops a new mode that utilizes active power reserves of hydro-photovoltaic complementary bases to ensure frequency security in long-distance transmission. Firstly, an active power-frequency coupled active support control strategy for the hydro-photovoltaic complementary system is proposed, and models of photovoltaic virtual inertia and hydropower frequency response are constructed. Then, taking the cross-regional sequential control signal of the DC system as a disturbance, a frequency response model of the sending-end power grid that takes into account the frequency regulation capability of hydro-photovoltaic complementation is established. The external transmission capacity of frequency regulation active power is quantitatively analyzed, and parameter sensitivity analysis is performed. Finally, simulations are carried out using MATLAB/Simulink. The results show that under the hydro-photovoltaic complementary mode, the nadir frequency of the sending-end power grid is significantly improved, with a steady-state frequency deviation of –0.190 Hz, which is superior to the –0.517 Hz observed with thermal power alone. Increasing grid damping and inertia can mitigate frequency drops; however, due to the dependence of photovoltaics on solar irradiance, their long-term active power modulation capability is limited. The conclusion indicates that large-scale hydro-photovoltaic complementary bases can serve as new regulation resources for long-distance active power support, positively contributing to the frequency stability of the sending-end power grid and providing technical support for secure and stable cross-regional transmission of renewable energy.