It is difficult to perform dynamics modeling of tethered balloon system due to the flexibility of the cables and the complexity of the gust forces. In this work, we propose a novel modeling method for near-ground tethered balloon system to analyze its dynamic responses while considering complicated boundary conditions. The proposed dynamics model can describe the characteristics of the flexible cable when it suffers large translations and bending. A three-dimensional dynamics model to deal with the coupling of the tethered balloon system and flexible cables is developed based on the multibody dynamics theory. The dynamic responses of the tethered balloon system under complicated operating conditions are investigated. Finally, how the critical parameters affect the dynamic responses of the tethered balloon system is explored in details.The results indicate that the proposed dynamics modeling method of the tethered balloon system is effective. The motion responses of the system are the most stable when a 600 meter high tethered balloon has 320 segments of tethered cables under the same external loads.