An automatic wheel-driven generating device mainly includes a base fixedly mounted on a wheel around an axle thereof and provided with a centered hollow column and one or more radially extended hollow pipes for each receiving a light-emitting element therein; a winding reel enclosed between left and right magnetic poles and fixedly mounted on the hollow column; and a magnet holder having a cap screwed into a center thereof and being fixedly mounted on the axle. The magnet holder carries magnets and is associated with the winding reel and the magnetic poles without contact with them. When the wheel rotates, the winding reel and the magnetic poles on the base rotate along with the wheel relative to the magnet holder that is unmovably fixed on the axle, so that an induced current is produced to drive the light-emitting elements to emit light beams. Each of the light-emitting elements is associated with a light-guiding bar that is connected to the wheel and has an uneven outer surface, so that light emitted from the light-emitting element is reflected and refracted in the light-guiding bar, making the whole rotating wheel a bright light circle to ensure a rider's safety.