An optical touch device includes a light guide module including a first light guide component, a second light guide component, a third light guide component and a fourth light guide component, a light source module including a first light emitting component and a second light emitting component for emitting light alternately and an image detecting module disposed between two neighboring ends of the first light guide component and the second light guide component. The third light guide component and the fourth light guide component are mirror light guide components. A field of view of the image detecting module covers the third light guide component and the fourth light guide component. The optical touch device can avoid a blind zone and can be used as a dual-touch device or a multi-touch device. A locating method and a linear source module are also provided.