To build a ground simulation experiment system for an in-orbit assembly, based on cold gas propulsion, we design a three-degrees-of-freedom free-flying robot simulator, and analyze the structural design, gas path system, dynamic modeling, and control system of the simulator. First, we adopt a modular design to partition the main structure of the simulator for different functions. Second, according to the working principle, we analyze and verify the bearing capacity of the simulator through an experiment. Subsequently, we arrange the nozzle in a partially decoupled way with the entire air path system, which is further designed. Then, we analyze the factors that influence the thrust size of the nozzle theoretically and verify them experimentally. Finally, we use the Newton-Euler method to establish the dynamic equation of the simulator. Simultaneously, combined with Simulink and Adams, we build a control simulation model and perform a motion simulation. The experimental results show that the simulator can carry a weight of more than 800 kg, with a force of 8 N on a single side and an overall running time of 30 min. Through simulation, we observe that the simulator has a good tracking effect on the reference input. The designed simulator can provide a mobile carrier for ground experiments of a super-redundant modular manipulator.