A modular robot joint for fitting easily through small openings is presented, which has a rotational degree of freedom. In addition to the common features of conventional modular robots, each module can maneuver through small openings and into an enclosed space to maintain areas that are not accessible to larger modular one. And it has a high load carrying capacity and torsional capacity compared with the universal robot. Several modular joints can access successfully the small openings, structured enclosed space in order to provide transportation and load carrying capacity for the maintenance operations. In this paper, the structure design of modular joint, static analysis, load running experimental verification and simulation through small openings are illustrated in detail. Finally, the research on modular joint is summarized and analyzed.