A high penetration photovoltaic distribution network contain numerous voltage-sensitive power electronic devices, which effect with each other, increasing the computational complexity of the node grid-connected critical capacity. Therefore, an analytical method for calculating the grid-integration capacity of distributed photovoltaic devices at feeder injection points is proposed, using a single-port reduced-order model, while analyzing the interactions effect between connected devices and the injection point. Firstly, based on the current injection balance equation of the distribution network, the grid characteristic function is defined to characterize the nonlinear relationship between the current and voltage at the infeed point. A single-port reduced model is used to simplify the grid characteristic function to reduce the computational complexity. Then, the equivalent parameters of the model are calculated by selecting some specific cross-sections of the grid characteristic function. The grid-connected critical capacity expression at the infeed point and quantify the interaction effect is analytically derived. Finally, a case study on the IEEE 33-node distribution network shows that the grid-connected critical capacity is improved to a greater extent when the photovoltaic equipment is connected to the adjacent location of the infeed point. Among the different distribution network topologies, radial distribution network has the strongest effect of access equipment with the infeed point, followed by the ring distribution network and the smallest in the two-terminal power distribution network.