Abstract: The settlement and inclination of the pier of existing railway bridge is caused by the upper long-term load, which has been affected the running safety. Geological exploration found that a large karst cave is located under thebridge pier foundation. Based on the catastrophe theory, according to the instability conditions of cave roofs thebearing capacity of the original piles was calculated. On this basis, consolidation strengthening was required for thefoundation by using the method of adding piles and enlarging bearing platform, according to the distribution of karstcave and the geological characteristics. And the proper length of adding piles was provided by theoretical calculation.What is more, the relationship between the length of adding piles and the settlement and inclination of the pier wasanalyzed, based on the numerical simulation software. The results showed that the numerical calculation resultsagreed with those of theoretical calculation. The bearing capacity of the original bridge pier foundation could not meetthe design requirements. The proper length of adding piles was 1 4. 5 m. The settlement and the maximum settlementdifference in lognitudinal direction of the bearing platform with the proper length of adding piles was 6. 9 mm and0. 89 mm respectively under the upper loads.