Research conclusions: The design concept and construction requirements of large cavity in tunnel are largely different from the tunnel in other places． The crossing rationality，the stability of fundament，the safety of structure and the feasibility of construction must be considered specially． There must be no any hidden trouble． The morphological features of cavity and the fillings must be checked out and meanwhile，the related karst geological condition and hydrogeological condition must be explored gradually． And after，the success can be achieved by taking the relative measures．