Aiming at the requirement that the operation tasks that cannot be completed by one person alone need additional assistance, the modular and reconfigurable supernumerary robotic limb is proposed and developed, which is composed of several basic modules connected in series with the same structure and function. In order to realize the module connection configuration and the shape reconfiguration capacities of the modular supernumerary robotic limb, the structure and function requirements of basic module units are analyzed. According to the requirements, the module unit based on steering engine driving is designed. Each unit is composed of two wedge-shaped sub-modules rotating coaxially with each other to form a rotational degree of freedom. The mechanical connection mechanism based on magnetic attraction is designed, which can realize the quick connection and separation of two modules. Furthermore, the connection method using double row metal contacts with interlacing disalignment is proposed to ensure the reliable electrical system connection of two module units. Four different connection orientations of two modules connected in series are clarified. The relationship between the configuration types of the supernumerary robotic limb and the number of modules is obtained. Facing to the supernumerary robotic limb with arbitrary number of modules and different connection orientations, the general forward kinematic model of the modular supernumerary robotic limb based on virtual axis is established. Experiments were carried out aiming at the rotation, carrying and fast connection capacities of basic modules, as well as the abilities of assisting the operator to carry wire and deliver telephone of supernumerary robotic limb in welding process, which verifies the assistant ability of the proposed new modular supernumerary robotic limb.