Abstract: The stratum at Luanchuan Molybdenum Mine is hard,broken,easily leaking and absolutely complex,so that the casing drilling was carried out in exploration tasks.However,the cases during drilling were usually broken at their thread.FEM is used to analyze the tensile strength and fatigue strength for the sake of understanding what makes those cases invalid.The results indicate that the fatigue failure caused by large number of impacts force into the thread is the essential cause to make the thread broken.Then an optimal scheme is put forward,and the new designed cases are applied in field drilling to solve some difficulties.

Key words: Thread, Fatigue failure, Optimization design