Abstract: In order to reduce the axial tensile force on the pipe string in deep and ultra-deep wells, and to reduce safety factor, it is often necessary to configure a telescopic tube in the pipe string. However, when checking the strength of the pipe string, the safety factor of the pipe string often occurs abrupt change. Due to the lack of mechanical analysis methods for the pipe string, it is impossible to make an accurate judgment on the safety of the pipe string, which affects the safety of the completion pipe string for oil testing. The full-scale finite element model of the telescopic tube was established by using the finite element analysis software, and the loads and stresses on the telescopic tube under working conditions, such as setting, perforation, fracturing, well opening, and well shut-in, and extreme conditions were analyzed. And the maximum load on the telescopic tube under extreme conditions corresponding to five typical working conditions was given. According to Lame equation and strength theory, the method for calculating the strength of the telescopic tube under complex load was derived, and the stress value of the telescopic tube under typical and extreme conditions was calculated. The results show that: in deep and ultra-deep wells, the stress value of the telescopic tube is the highest under fracturing condition, which is the most unfavorable condition; while under the conditions of setting, perforation, well opening and well shut-in, the safety factor of the telescopic tube is usually above 1.35. The combination of internal, external force and axial force on the telescopic tube under extreme working conditions corresponding to five typical working conditions was obtained, which provides a load combination template that should be avoided in actual operation. The theoretical analysis cannot take into account the existence of the keyway of the telescopic tube, nor the influence of stress concentration. The stress obtained by the theoretical analysis is about 15% lower than that of the numerical analysis. The established safety evaluation method for telescopic tube under typical working conditions may provide a theoretical support when analyzing the safety effects of telescopic tube on pipe strings.