Abstract: Tube strings sometimes broke in the Tahe Oil Field. Finite element simulation was applied to study tube string accidents, and found out that the radial depth of cracks is the main factor controlling tube string breaks. The critical lift force of different defect depth was concluded, which provided reference for site operation and tubing check. According to the analysis of thread turns, the contact stress distribution of thread engagement is of "saddle" type, and in high stress on both ends of the threaded connection, prone to fastening. By combining simulation and field experiment, we can effectively reduce tube string failures.