Abstract: The feed support of the FAST telescope is a rope-driven parallel manipulator with huge spans. In such mechanism, an airborne focus cabin carrying the feed receiver searching for radio waves is suspended by six parallel steel ropes and can move slowly in a large range, a spherical focus surface, through the continuously coordinated coiling/uncoiling of the six ropes. Because of the large sectional areas of the steel ropes, the gravity cannot be neglected in the equilibrium analysis of the mechanism. Moreover the power supply cables and optical communication cables also have to be suspended on the ropes for the electronic equipments in the cabin. So there are quite complicate interactions among the workspace of the parallel manipulator, how the airborne focus cabin poses itself, the rope forces and rope geometries. The equilibrium equations are formulated in the paper for the steel ropes and the focus cabin respectively. The equations of the focus cabin are solved by introducing an optimization principle of intending a uniform allocation among the six rope forces. As result, the optimal rope forces and the optimal pose of the cabin are computed along the whole focus surface. The maximal and minimal rope forces are therefore deduced from the optimization solutions of the six rope forces as well as their variations in the focus surface. Combining the optimization solutions of the cabin pose with the required tilt of the cabin at a focus point, we calculate the minimal workspace of the AB rotator, a tilt compensator in the focus cabin. Finally, the tower height and the gravity center of the cabin are studied on how their changes influence the optimization solutions of the rope forces and cabin tilt. The investigation proves that the maximal rope force is sensitive and inversely proportional to the tower height, whereas the cabin tilt is sensitive to gravity center of the cabin. They both should be optimized in the future design of the FAST telescope.