A mobile device protecting structure includes a first frame having a first frame portion and a first guard plate, a second frame having a second frame portion and a fixing edge, and the second frame portion encircling the first frame portion, and a packing element disposed in the second frame portion and inserted into a packing slot formed at the exterior of the first frame portion, and a waterproof ring installed between the first and second frames and having an internal periphery. A mobile device is covered by the first and second frames, and a second guard plate is selectively packed between the internal periphery and the fixing edge or packed between the internal periphery and the mobile device depending on the thickness of the mobile device.