A composite simple plug includes an insulating case, a plurality of terminals, a plug for an Internet line or a telephone line, and a fastener. The insulating case has a trough. The terminals are located in the insulating case and are exposed to the trough. The connector is located at the front end of the insulating case and electrically connected with the terminals. The fastener is located near the front end of the insulating case. The insulating case is suitable for a variety of plugs. The composite simple plug can be connected with a plug for an Internet line, or is connected with a linking element. The converting method is simple. The fastener firmly fastens the plug for an Internet line or the linking element so that the plug for an Internet line will not come away from the composite simple plug.