M3 Express can move independently and use three dual-purpose wheels, which serve as driving wheels and mechanical inter-connectors to build complex systems [15]. HEX-DMR II employs homogeneous modules and docking mechanisms for self-assembly that can be used to replace individual modules to alter its practical capabilities or replace damaged modules for self-repair [16]. In these mobile robots, joint actuation, mechanical connection and perception abilities are important for docking and self-assembly. Although an individual may be small or simple, when these individuals are aggregated into a system, these high-level structures become qualified for more tasks. A thousand small swarm robots can cooperate through local interactions in a collective way to form different shapes [17]. Tibet is a small-scale robot with a simple design but it operates collectively to manipulate large objects and overcome obstacles [18]. Without central control, a practical robot system employs a loosely coupled mechanism and stochastic low-level components to achieve robust high-level behaviors [19]. A swarm of identical low-cost quadruped robots were combined with a magnetic docking mechanism and used self-assembly to link to each other in a chain, responding to challenging for individuals such as obstacle navigation and object transporting [20]. SMORES-EP modular robots have been combined with visual sensors to enhance their perception capacity and high-level planning, and local docking and motion were integrated to reactively reconfigure the robot to adapt to different environment types [21]. Swarm robots with mechanical interconnections have another advantage of combiningtools created by humans and bio-inspired schemes. Combining wheeled mobile units and joints in a chain, snake robots consisting of reconfigurable segments can traverse diverse environments [22]. In addition to robots on the ground, a number of self-assembling surface robots [23; 24] and aerial robots [25; 26] with interconnection capabilities have been proposed. In a previous work, a modular robot system [27] was proposed to build a variety of bionic structures and an experimental demonstration [28] was performed. According to the docking method, a self-assembly planning algorithm based on local behaviors was presented [29]. However, the mechatronic design can be furtherly optimized for a compact structure such as a snake-like configuration and mobility can be extended to fit more kinds of terrain. In addition, the perception in that case was poor, using only infrared sensors, and joint actuation was insufficient for the assembled structure to attain multiple DoFs of locomotion. In this study, we aimed to propose a novel self-assembling mobile modular robot (SMMRob) which has better joint actuation with long links and rich visual perception capacities, as well as a well-implemented system. The potential applications include planet exploration, disaster rescue, or field exploration. In these extreme and hostile environments, modular robots can operate in a swarm fashion for wide-area coverage and connect into a multi-joint robotic structure with better terrain adaptability to travel through territories, or a swarm of modular robots can dock with a body platform equipped with a communication radar or solar array in order to advance. The main contributions of our work are as follows. * A novel parallel mechanism-based joint actuation and onboard vision-based perception system are presented to enhance docking, self-assembly, and actuation for the joints' DoFs. The parallel mechanism can make the robot thinner and enlarge the workspace by longer providing links, which is different from revolute actuators. On-board vision-based perception improves sensing information and enables the robot to sense autonomously, rather than using external cameras. * The docking methods for a multi-stage docking procedure based on relative positioning in different ranges are proposed for self-assembly.