In order to solve the problems of high-frequency and high-amplitude vibration and easy deviation of drilling holes with rotary drilling at increased WOB under special working conditions， such as brittle rock， soft and hard alternate formation， this paper analyzes the rock crushing mechanism of rotary drilling rig to find out the causes of the drilling problems at increased WOB. Study was conducted on the drilling process at increased WOB with the implementation plan and applicable scope put forward. Field application of the process was performed to verify its feasibility， and the results showed that rotary drilling at reduced WOB was of great significance to reduce drilling vibration， improve the structural stability of equipment and ensure the borehole quality.