Super-large karst caves are encountered during construction of Naqiu Tunnel on Yongji Highway.As a result,a combination structure of bearing pile foundation + vertical and horizontal frame-beam-plate structure + reinforced concrete retaining wall and slag backfilling are adopted based on characteristics and treating difficulties of large karst cave and theoretical and site studies.And then a set of construction methods of alternate up-down construction of the structure on basis of slag backfilling is established.The construction practice shows that the technologies used are rational and feasible and good effect has been achieved.