A large number of distributed power sources are connected to the distribution network, which changes the single-side power supply structure of the traditional distribution network, resulting in bidirectional currents, voltage overshoots, and tidal fluctuations in the distribution network. Firstly, the fault characteristics of inverter-type distributed power supply based on constant power control and the fault characteristics of active distribution network are studied. An active distribution network differential protection scheme with adaptive amplitude ratio is proposed. The phase difference of the terminal current is used to adaptively adjust the amplitude constraint of the protection. When the fault occurs in the zone, the braking range is reduced to prevent the protection from being rejected. In the case of an out-of-zone fault, the braking range is increased to prevent the protection from malfunctioning. Finally, the simulation model is built in PSCAD to verify the proposed protection scheme. The simulation results show that the protection scheme has good adaptability.