The first excursion failure criterion was introduced to calculate the dynamic reliability of drill string elements. Considering the random of geometry size and mechanical property parameters of drill string in deep & ultra-deep wells, the drill string system was dispersed into several basic elements according to the structure, strength and differences in the nature of formation rock in the well depth direction. And the overall dynamic reliability was quantitatively calculated through the serial theory. Through an example, the results show that the dynamic reliability of the drill string elements appeared fluctuation near the depth of 4500 meters for the inside worn of the drill string and casing, and uneven loading of drill string. The dynamic reliability of drill string in deep and complicated formation was less than any other drill strings. The drilling pressure, rotation speed and drilling depth were the main influencing factors on the dynamic reliability of drill string.