There is provided a smoke detector including a light source, a reflective surface, a light sensor and a processor. The light sensor receives reflected light when the light source emits light toward the reflective surface, and generates a reference detection signal when there is no smoke. The processor receives the detection signal from the light sensor, and automatically selects a set of predetermined condition thresholds according to a profile of the detection signal to be compared with the detection signal thereby determining whether to generate an alarm according to the comparison result.