The invention discloses a method for forming a pile in a stratum with a hard rock stratum in a lower part. The method comprises the following steps: drilling by adopting a rotary drilling rig, and stopping drilling construction when the rotary drilling rig reaches the hard rock stratum; continuing drilling to a designed elevation by adopting a water mill drilling rig; cleaning the bottom of a hole, placing a steel reinforcement cage into the drilled hole, and then filling pile body concrete. According to the method, vibration and noise pollution caused by hard rock impact and blasting are avoided; a secondary hole cleaning link after the hole is finished is also omitted; the problems of hole bottom sludge and difficulty in loosen soil cleaning are solved; respective advantages of the rotary drilling rig and a manual hole digging pile are fully played; the construction period can be effectively shortened; the construction cost can be reduced; the construction quality can be ensured; the method is not limited by ambient environment; the requirements of safe, civilized and green construction are met.