Abstract: An inflatable device with air bag includes an air bag, two first assembly portions and two second assembly portions. The two first assembly portions are respectively fastened at two sides of the air bag, and the two second assembly portions are respectively fastened at another two sides of the air bag. The two second assembly portions correspond to the two first assembly portions. Thereby, by utilizing the assembly portions to repeatedly assemble the air bags, the dimension of the inflatable device with air bag is quickly expanded to a proper size to wrap the objects with different sizes. It prevents the object from being impacted, and achieves the waterproof effect and the moisture-proof effect. A packaging method by using the inflatable device with air bag is also disclosed.