Abstract: A transportation device for an offshore platform, including a vessel and a floating structure which are fixedly connected. The floating structure is placed on a sea surface and is configured to assist the vessel to sail. The floating structure is provided with an adjustment mechanism which is configured to adjust the floating structure to rise and fall relative to the sea surface. A rail is arranged on the vessel and is in sliding connection with the topside module, so that the topside module slides onto the vessel from land. During the transportation of the topside module, the buoyancy of the floating structure is adjusted through the adjustment mechanism, so that the floating structure provides sufficient anti-rolling moments beside the vessel, thereby reducing the vibration of the topside module caused by the winds and waves during the sailing and reducing the potential damage to the topside module.