The installation of passive protection nets is an effective measure for preventing and controlling rockfall disasters on slopes. Designing passive protection nets that are effective, accurate, and economical is of great significance for enhancing the comprehensive disaster prevention and mitigation capabilities of the society as a whole. To address frequent rockfall disasters, a discrete element-based design method of passive protection nets for slope rockfall disasters is developed. By establishing the detailed discrete element models of the slope, rockfall, and passive protection net, indicators such as rock movement trajectory, number of intercepted rocks, and interception efficiency are analyzed to determine the parameters of the passive protection net, such as height and length. Then the type suggestion of the passive protection net is put forward based on the monitoring of the speed and impact energy changes of the rockfall. According to the analysis of the deformation variation of the protection net on various positions of the impact cross-section, the maximum deformation position of the passive protection net under the best protective condition is determined. Taking the construction of a reservoir road at a large hydropower station as an example, the discrete element-based design method for passive protection nets is used to quantitatively analyze rockfall disasters, and it is determined that the optimal length of the passive protection net is 30 m and the height is 5 m, with which the gravel interception efficiency can reach 92% and the slope material is capable of withstanding an impact of 108.5 kJ. Based on the above analysis, the discrete element-based design method for passive protection nets for slope rockfall disasters can quantitatively evaluate the impact scope and degree of rockfall disasters, so it can provide a new approach and some reference for the design of rockfall prevention and control measures in engineering projects such as water conservancy and hydropower stations, roads, and bridges.