During exploitation of extended reach well (ERW) drilling engineer, the problem of drill string failure is very serious; drill string vibrations are main reasons. In this paper drill string of extended reach well of different part is object being studied. The models about various vibrations are set up. Modal vibrations of drill string are studied. All sorts of resonance frequencies are obtained practicability. This study proves basic method for optimizing drill tool of extended reach well and reducing drill string failure.