Two large karst caves containing confined water were encountered during drilling of ZK27A07 borehole for Baijian iron ore exploration in Shahe City， Hebei Province. In the placement process of ?89mm casing， the casing unscrewed and fell downhole. In the process of accident treatment， the casing fell downhole again， with fishing tools breakingoken and falling downhole for several times， forming a superposition of multiple movable accident heads in the karst cave. We successfully handled the casing falling accident by casting concrete and natural by-pass drilling， so as to avoid scrapping the drilling hole. In this paper， the whole process of accident treatment is described in detail. Through careful analysis of the process and causes of the accident， the specific method is presented for safe placement of the casing string with excessive length and overweight. At the same time， how to realize reliable borehole wall sealing with the casing string in drilling through multiple karst caves containing confined water is discussed， which is intended to provide reference for similar projects.