[Objective] In response to the insufficient distribution network support and altered voltage-frequency coupling characteristics caused by the reduced inertia of the main grid due to large-scale renewable energy integration，this paper studies the transient voltage-frequency coupling mechanisms in distribution networks with distributed photovoltaic （DPV） integration. It aims to provide a theoretical basis for the development of an optimization and control strategy for the stable operation of distribution networks with high renewable energy penetration. [Methods] Based on the IEEE 33-bus system，the transient voltage and frequency response characteristics before and after DPV integration were analyzed. It was found that the dynamic coupling between voltage and frequency becomes more pronounced during renewable energy-dominated transients，with frequency issues being more critical under low-inertia conditions. The underlying causes of transient voltage sags and frequency fluctuations were explored，clarifying that the interaction between DPV and the network is a key factor in voltage-frequency coupling. From the perspective of power balance，the coupling mechanism between voltage and frequency during transients was elucidated by quantifying the dynamic relationship between active power variations in main grid and distribution networks. [Results] Simulations demonstrated that the transient response characteristics of DPV dominate voltage changes and further disturb frequency during transients. In low-inertia systems，the low-voltage ride-through （LVRT） process of DPV not only determines the voltage sag and recovery but also indirectly disturbs system frequency through its reactive power support. During transients，voltage changes alter the active power supply-demand relationship on the distribution network side，disrupting active power balance and leading to frequency fluctuations through power interactions between main grid and distribution networks. This clarifies the dominant path through which voltage changes disturb frequency. [Conclusion] This paper quantitatively describes the coupling relationship between voltage and frequency and reveals that the coupling mechanism is essentially a dynamic transmission process of "voltage fluctuation→active power imbalance→frequency response". The reactive-active power coupling characteristics of DPV exacerbate the complexity of voltage-frequency coupling. The clarification of this mechanism provides a theoretical basis for the development of voltage-frequency coordinated control strategies to suppress frequency fluctuations.