Abstract: The safety and reliability of the completion test string for deep high-temperature and high-pressure oil and gas wells is of great significance to ensure the safe and efficient development of deep oil and gas. Considering the comprehensive influence of factors such as temperature and pressure changes, pipe end constraints, and buckling friction during the test, the force analysis model of the test pipe string was established, and the mechanical analysis software of the completion test pipe string for deep high-temperature and high-pressure gas wells was developed. Then, the temperature and pressure analysis, stress deformation calculation, and mechanical strength check of the test string in a well in Shunnan area were performed, which revealed the reasons for the failure of well completion test string. The results show that: the developed software can more accurately analyze the temperature and pressure environment for the test string in the high-temperature and high-pressure wells, and can perform stress deformation analysis and strength check, which can be used in engineering practice; the local corrosion damage and the generated cracks on the surface of the test string in the well will reduce the strength of the pipe string, which is the main reason for its failure; special attention should be paid to the effect of corrosion on the mechanical strength of the pipe string in high-temperature and high-pressure wells in this area. This research can provide a theoretical basis for the optimal design and safety control of the completion test string in high-temperature and high-pressure wells.