An exemplary streetlight system for illuminating a road surface comprises an illuminating device, an infrared detection device and a controlling device. The illuminating device includes a light source. The infrared detection device is configured for detecting whether there are vehicles or passersby on the road surface and generating a detection signal. The controlling device is configured for receiving the detection signal, generating a controlling signal according to the detection signal and sending the controlling signal to the light source, thereby modulating the brightness of the light source.