Abstract: In order to apply semiconductor laser to measure cavity loss with continuous-wave cavity ring-down technology,the laser diode's driving circuit was designed.The circuit was composed of different modules,such as modulation,driver and temperature controller.Based on theory of passive cavity's multi-beam interference,modules' parameter demand was suggested after analysis and numerical simulation of ring-down signal.The measurement system of cavity loss was built with the circuit.Under the guarantee of the semiconductor laser trouble-free,tests indicated that the shutoff time of laser diode was about 60ns and the precision of temperature controller was better than 0.01℃.The measuring accuracy was better than 1% for a cavity whose single pass loss is about 50×10-6.Results indicate that the circuit meet the theory design demands and can be applied in high-accuracy measurement of cavity loss.