In this paper, we propose a tandem tethered balloon scheme as a solution for increasing the operating altitudes of balloons to near-space altitudes of more than 20 km. By adding a middle balloon with a smaller volume than that of the top balloon in the middle weak wind zone, this scheme partially offsets the weight of the tether above the middle balloon, thereby greatly increasing the operating altitude of the tethered balloon. Another significant advantage of this scheme is that, when the working altitude exceeds 20 km, the opposite directions of the winds at high and low altitudes can be exploited to reduce the horizontal displacement of the top balloon, raising the safety factor and the wind-resistant capabilities of the tether. In this paper, a three-dimensional static model is established for the tandem ultra-high-altitude tethered balloon, and a static equilibrium governing equation of the tandem ultra-high-altitude tethered balloon is derived. In addition, a design method based on the principle of the tether safety factor has been proposed for the tandem tethered balloon. For the first time, a sensitivity analysis was conducted for the tether resistance coefficient, balloon resistance coefficient, and balloon lift coefficient for the design optimization and flight test stages. Finally, we analyzed the three-dimensional cross-sectional profile of the tether, the maximum tension of the tether, the length of the tether, and the variation of the angle between the ground end of the tether and the horizontal plane for a tandem tethered balloon system that remained airborne continuously for one month. This work confirms that the model is viable in complex and variable wind field environments and capable of long-duration flights. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.