The Gaoshan Tunnel on Qianjiang-Zhangjiajie-Changde Railway passes through a giant karst cave at section DIK53+678. In order to safely and successfully cross the giant cave, the stability of the karst cave is studied by geological survey, UAV detection, deformation monitoring of dangerous rock mass, and vibration velocity monitoring and crack development monitoring of dangerous rock mass. The results show that dangerous rocks are widely distributed in karst caves, and there are many rockfalls around the cave wall; hence, effective evasion and necessary safety protection should be provided. Various calculating methods of rockfall impact force in China and other countries are summarized and analyzed. The rockfall load, reaching 442.1 kN, is calculated by using the method recommended in Railway Engineering Design Technical Manual: Tunnel, which is regarded as design reference for temporary construction protection. Based on the calculation results of the rockfall impact force, a triangular steel structure protective shed frame is proposed to against the impact of the rockfall in karst caves. After structural optimization, the protective shed frame can resist the impact load of 0.7 t falling rock from 50 m height, which can meet the safety protection requirements of the giant karst cave.