The invention provides a method for using a rotary drilling rig to drill into a hard rock stratum. The method for using the rotary drilling rig to drill into the hard rock stratum includes following steps: 1, designing a bottom elevation of a pile; 2, confirming a boundary line between a soft soil layer and a hard soil layer; 3, reaming the holes through a pinch roller drill barrel d2 in diameter and h2 in height; 4, reaming the holes through a pinch roller drill barrel d3 in diameter and h3 in height; d5, reaming the holes through a pinch roller drill barrel d4 in diameter and h4 in height; 6, sequentially repeating the steps from the step 3 to the step 5 until drilling to the bottom elevation of the pile; 7, using a short cutting tooth slag grasping drill d1 in diameter and h1 in height to remove slag, wherein d2 is less than d3, d3 is less than d4, d4 is less than or equal to d1, h1 is less than or equal to h4, h4 is less than or equal to h3, and h3 is less than or equal to h2. The method for using the rotary drilling rig to drill into the hard rock stratum is simplified in process, uses a decompression drilling method during the drilling process to guarantee drilling perpendicularity, and resolves the problem that the hard rock stratum is difficult to core and obviously improves work efficiency by successively using three drilling steps from the small diameter to the large diameter during the drilling construction process.