Abstract: There is a lack of design theory of passive flexible shield structures. In this paper, the complex structure was transformed to simple mechanical model, and the stiffness increment variation method was used to investigate the mechanical behavior of steel gantries of passive flexible shield structures entering the ultimate state. The yielding mechanism of steel gantry and steel rope was established, and the absorbed energy of the steel rope and the ultimate bearing capacity of the steel gantry were studied. The calculation theory of energy dissipation capacity of the whole structure was established, which leaded to design theory of passive flexible shield structures based on the energy method. In order to validate the accuracy of the design theory, the theoretical results were compared with finite element results. Based on the above design theory, suggestions were given about design and type selection of passive flexible shield structures. Research shows that theoretical results are in good agreement with the finite element results. For type selection, in order to improve energy dissipation capacity of the whole structures, steel rope should yield firstly and its length and yield force should be increased. Parabola steel gantry should be used preferentially because it has highest ultimate load capacity under the same conditions.