The geological conditions of the huge waterrich karst cave under the floor at YK37+930 of a tunnel in Nanyang is forecast by means of advance geology prediction, exploration drilling and labor investigation. The hydrology, geomorphology and sinkhole distribution are investigated. Countermeasures, including ground surface sinkhole blocking and arch bridge crossing, are proposed, on basis of the data collected by investigating and studying on the hydrology, topographic feature and sinkhole distribution. The key points of the treatment of the waterrich karst cave are presented. The operation and monitoring of the tunnel show that the treatment technologies for the karst cave are rational and effective. The paper can provide reference for similar projects in the future.