A method, a device, and a system for protecting parallel-connected topology units are provided. A target signal transmitted via the signal synchronization line is obtained, and the target signal is sent to other controllers based on a type of the target signal. If the target signal is a carrier synchronization signal, the current power module is controlled to perform carrier synchronization and to be in a working mode. If the target signal is a power module fault signal, the current power module is controlled to be in a shutdown mode. The signal synchronization lines between the parallel-connected topology units are shared in a time-sharing manner, where the carrier synchronization signal is transmitted if the power module works normally, and the power module fault signal is transmitted if the power module is faulty. The topology units monitor the transmitted target signal in real time for fast synchronous protection.