Abstract: The axial vibration of drill string can cause the drill bit failure, drill pipe wear and increase the cost of drilling. The paper created the finite element model of drill string axial vibration by ANSYS. Then the natural vibration characteristic and dynamic response are analyzed by Block Lanczos method and Full Matrix method, so the relationship between drilling fluid and drill string vibration frequency was known. The natural vibration frequency and amplitude of drill string increase with the decreasing of drilling fluid density. The change trend of drill string response force is the same with response displacement. When the density of drilling fluid is decreasing, both of response displacement and force are increasing which cause rising of the von Mises stress and reduction of fatigue life.