A light source module arranged in an image scanning device having a scanning platform and an image-pickup module for scanning a transparent object is disclosed, The light source module includes a cover, a light source mounted in the cover for emitting light to the scanning platform for scanning a transparent object, a RGB filter mounted between the light source and the scanning platform for filtering off the light into RGB lights, a switching mechanism connected to the RGB filter for being controlled to alternately allow one of RGB lights to be transmitted to the scanning platform, and a transmission mechanism for carrying the light source, the RGB filter, and the switching mechanism to move with the image-pickup module for accomplishing the scanning operation to the transparent object.