From May 15,2014 to September 1，the No.19 and No.20 piers of Gantangjiang super major bridge at Guiyang-Guangzhou railway had experienced abnormal subsidence with the downward displacement of 50.6-52.2 mm and 12.4-16.3 mm, respectively. In order to understand the reason of the subsidence and to propose appropriate measures to the problem of bridge pier displacement, mobile drilling, in situ and laboratory geotechnical tests were completed. Field observation and testing data jointly shew that the agelong heavy rainfall condition and subsequent underground karst water fluctuation caused karst collapse occurred at the underlying strongly karstified limestone. This was the main reason of geological impact on the subsidence of No.19 and No.20 piers with friction pile foundation and the pile length of about 20 m. For engineering remediation, end bearing pile foundations were respectively added in the both bridge piers. After about 1 year’s displacement monitoring, the subsidence of the piers has been properly controlled, which suggest that the reinforced measurement is safe and reliable; and the end bearing pile foundation is generally suitable when other railway bridges are carried out in the similar case of deep covered strong karst limestone area.