Fatigue is one of the reasons for drill string failure. Based on the static stress characteristics, fatigue ratio was usually used to assess the risk of drill string fatigue failure, but it only takes the static stress into account. In this paper, by taking into consideration of the dynamic characteristics including stress and its change frequency, fatigue frequency ratio is proposed to reflect the risk of dynamic fatigue failure for drill string under actual drilling conditions. Through the dynamic calculation of drill string based on actual wellbore trajectory, the dynamic stress characteristics and fatigue frequency ratio are obtained. The results show that fatigue frequency ratio not only reflects the potential impact of well path's features, drill string structure and bit weight on the fatigue failure, but also reflects the effect of the dynamic stress and its frequency variation on the fatigue failure. By analysing Well X whose drill string suffered leakage badly, it proves that the fatigue frequency ratio method can effectively predict the position and risk of fatigue failure in the drill string.