Provided is a drilling apparatus, comprising a PHC pipe cased pile having a drilling section for containing fluid and a hammer element arranged inside the cavity of the PHC pipe cased pile which moves along the drilling direction till the bottom of the drilling section. As the hammer element moves along the drilling direction till the bottom of the drilling section, a localized hydraulic force will be induced in the fluid contained in the drilling section of the PHC pipe cased pile by a sharp rise in the fluid pressure in addition to the impact force directly acted on the rock and soil by the hammer element. Under action of the hydraulic force and the impact force, the surrounding soft rock stratum could be crushed and the rock stratum with higher strength is left intact, that is, the drilling apparatus proceeds until harder rock. At this point, concrete casting will bond with the harder rock, resulting in a better bearing capacity of the PHC pipe cased pile foundation.