The Gaoshan Tunnel on QianjiangZhangjiajieChangde Railway obliquely passes through a giant karst cave at high level. The karst cave has super large size, poor overall stability, and many dangerous rocks distributed on the cave wall. Rock fallings caused by the displacement of dangerous rocks occur from time to time, which makes the treatment of the cave very difficult. Hence, 3 kinds of treatment schemes, i.e. line avoidance, backfill treatment and bridge span treatment, are put forward. Considering the factors such as construction, operation safety and economy, backfill treatment is selected, and the scheme of "backfill cave ballast + upper grouting reinforcement" is finally determined. During the construction, a series of key construction technologies, i.e. backfilling by stages, safety protection of bolt mesh spraying on the wall and roof of the cave, grouting reinforcement of backfills, protective construction of the structure of the tunnel and settlement pretreatment after construction of backfills, has been formed. The online settlement monitoring and construction process control of the superthick backfills show that the process of cave treatment is smooth, the treatment technologies are feasible, and the cost is rational. The results can provide reference for similar project in the future.