Expansion box, also known as expandable container room, expandable container room, expansion box or butterfly box, is a modular design folding prefabricated building product. This product is based on standard containers or light steel structures as the basic framework, using folding and expansion technology to maintain a compact folding shape during transportation. After arriving at the site, it is unfolded to the side or both ends through hydraulic systems, hinges, or mechanical devices, instantly expanding the indoor usable area to 2-3 times the original transportation size