Karst disaster is one of familiar geology disasters in tunnel construction in mountain area. The influence of concealed caverns on the stability of tunnel surrounding rock in tunnel construction in karst region is studied. The stability problem of tunnel roof under the influence of jumbo size concealed karst caverns is simplified to collapse destabilization problems of beam or plate. The analytical formula of the minimal safe thickness of karst tunnel roof is deduced on basis of elastic theory. The stability of tunnel roof in Wudu Reservior is analyzed by this formula; and the rationality and creditability of the formula is verified by in-situ test.