A new extension cord and its manufacturing method. The new extension cord has a plurality of sockets at intervals along its longitude direction so as to reduce the whole length of extension cord and increase the number of the sockets. The jacks of sockets are protected by caps to avoid occurrence of electricity shock and water entry into the sockets. The manufacturing method comprises steps of forming a plug, forming a plurality of sockets, forming electrical cords, disposed the sockets at intervals along a longitude direction of the electrical cord. The steps of forming each of the electrical cords comprise forming at least two electrical wires, forming an insulated wrap to enclose the conducting wires, welding the plug and the sockets to form an integral piece by ultrasonic wave.