Based on the geological exploration in the first and second railways of Xi’an to Ankang, the paper discussed the geological characteristics of karst phenomenon in the construction in the marine deposit extremely thick carbonate rock layer of carboniferous, Devonian and cambrian-ordovician, which developed in the hinterland of southern Qinling. According to the karst development degree and configuration revealed in the existing engineering construction, the karst was divided into vertical seepage karst zone and undersurface flow karst zone; and on this basis, it was put forward that different construction measures was applied for each karst type and part.