A ring-shaped LED lamp, including at least three LED chips and at least three pins partially located within a package body. The LED chips are connected in series and have two terminal ends connected to form a closed loop, and the pins are led out from connections between the LED chips. A lamp string includes at least one group of lamps and electrically conductive wires connected to multiple lamps, and each group of lamps includes a plurality of the above-described ring-shaped lamps. Also provided is a control circuit applicable for the lamp string. The control circuit includes an input device, a microprocessor, an output circuit, and a lamp string interface connected to the output circuit. The microprocessor receives a signal from the input device, and sends an I/O signal or PWM signal to the output circuit to trigger a voltage level change of the lamp string interface.