A network gateway device providing peer-to-peer proxy service is provided, including a P2P meta descriptor detector detecting an original P2P meta descriptor file from the public network, a P2P proxy control unit modifying the original P2P meta descriptor file to generate a modified P2P meta descriptor file, and forwarding the modified P2P meta descriptor file to a computer in the private network, an internal tracker receiving a first inquiry message, and replying with a pseudo sharing computer list, and a peer-to-peer engine loading the original P2P meta descriptor file to download shared contents, and forwarding the shared contents to the computer.