The access and application of phasor measurement units (PMUs) in distribution networks highly improve the system observability and further enhance the performance of operation control and energy management. A novel PMU-based estimation method of voltage-to-power sensitivity via network equivalent is proposed for distribution systems. An equivalent model is introduced to simplify the PMU-unobservable parts of the distribution network. The equivalent parameters are estimated with the PMU measurements on both sides. The original PMU-unobservable distribution network is simplified into a completely observable network with an arbitrary configuration of PMUs. The maximum correntropy Kalman filter (MCKF) is exploited in parameter estimation to handle the noise and bad data within the PMU measurements. The voltage-to-power sensitivity of the simplified system is then calculated according to the estimation of the equivalent network parameters. Case studies on the IEEE 33-node and the PG&E 69-node test feeders verify the correctness and effectiveness of the proposed method.