Humans who work long hours in weight-bearing jobs, such as soldiers and workers, often suffer from occupational diseases. These injuries can range from musculoskeletal injuries to neurological injuries and can have a significant impact on the workers. Various exoskeletons have been developed for strength enhancement, bringing great benefits to humans for load-carrying work and rehabilitation training. At present, common exoskeletons include lower limb exoskeletons, upper limb exoskeletons, elastic suspension backpack and so on. However, exoskeletons often have unavoidable defects such as interference with natural human movement and can only act under specific human gait. The Extra Robotic Legs (XRL) can help humans load heavy objects while largely avoiding these problems. The performance of human-machine collaboration will be poor because humans and the XRL are frequently controlled independently of each other. Additionally, the XRL may collide with humans and cause energy loss. Therefore, in this paper, a passive coupler consisting of a spring and a damper in parallel is designed to connect the human with the XRL so that the whole quadrupedal system can work better. Dynamically coupled twowheeled mobile inverted pendulum model and a Rimless Wheel model are chosen, and a Newton method is used to model the dynamics of the whole system. To analyze the human-machine collaboration capability of the system, the convergence characteristics of the coupler length for different coupler design parameters and initial conditions were examined using the Poincaré return maps generated from numerical simulations. The coupler parameters that make the regression curve converge fastest will also be determined, at this time the human-XRL system has the highest level of human-machine collaboration capability.