The present invention relates to a focus-adjustable device and a system thereof, wherein the focus-adjustable device is fabricated by forming a single closed flow channel in the internal of a transparent substrate, so as to make the substrate and the single closed flow channel together form a specific lens structure similar to the Fresnel lens. In the present invention, when a light illuminates on one side of the substrate, the light would be concentrated at a specific focus point distancing from the other side of the substrate after being deflected by the specific lens structure. Moreover, a fluid having a specific refractive index can be filled into the single closed flow channel by a flow channel opening, so as to carry out the purpose of adjusting the focus of the specific lens structure; or to make the light parallelly pass through the specific lens structure without any deflection.