A detecting device for detecting liquid or colloid, comprising: a light emitting device, configured to emit first light, wherein a first angle between a first emitting direction of the first light and a surface when the detecting device is located on the surface, wherein the first angle is larger than 0° and smaller than 90°; an optical sensor, configured to detect first optical data generated based on the first light; and a processing circuit, configured to determine if the liquid or the colloid exists in a predetermined range of the detecting device based on the first optical data. An automatic cleaner applying the detecting device is also disclosed.