An interface circuit, a string, and a system that are applied to power line communication, to lower device specifications, includes: an inverter, an optimizer group, a capacitor, a magnetic ring, power lines, and a signal line. An optimizer in the optimizer group is configured to adjust a size of a direct current output by a photovoltaic module connected to the optimizer. Two ends of the signal line passing through the magnetic ring are connected to the inverter. Two ends of a power line passing through the magnetic ring are respectively connected to the capacitor and the optimizer group. By using the foregoing interface circuit, a high voltage of a direct current output by the optimizer group is prevented from being introduced into the inverter, thereby reducing a specification requirement of a device such as a capacitor.