Research conclusions: (1)The main default of the karst subgrade of Luoyang—Zhanjiang Railway is the subgrade settlement．The penetration corruption caused by the fluctuation of underground water，vacuum absorptive corruption and pumping underground water by human being are the reasons for the settlement．(2)The treatment principles of karst subgrade are as follows：To the embankment，grouting will be conducted，to the road cutting，the principle of combining prospecting with grouting will be followed and the verification will be made by prospecting and boring，to the deep buried carst cave，the grouting will be conducted after flyash and mortar fillings，to the open or shaUow buried karst cave，the refilling will be conducted after excavation or the concrete cover plate will be added．(3)In the prospecting and grouting process，it is necessary to pay attention to the changes of grouting pressure and environment and control of grouting quality for getting perfect grouting result and good stability of the karst subgrade．