A ring-shaped LED lamp, comprising at least three LED chips and at least three pins partially located within an encapsulation member. 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 comprises at least one group of lamps and electrically conductive wires connected to multiple lamps, and each group of lamps comprises a plurality of the above-described ring-shaped lamps. Also provided is a control circuit applicable for the lamp string. The control circuit comprises aninput apparatus, a microprocessor, an output circuit, and a lamp string interface connected to the output circuit. The microprocessor receives a signal from the input apparatus, and sends an I/O signal or PWM signal to the output circuit to trigger a voltage level change of the lamp string interface.