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Abstract Based on the analysis of seismic intensity and randomness, an earthquake intensity probability model is built, and the relationship between seismic intensity and seismic acceleration is presented. The distribution law of casing parameters, including outer diameter, wall thickness, Young’s modulus and Poisson’s ratio, is described by analyzing the randomness of casing geometry and mechanical properties. With the setting up a mechanical model of the casing strings in earthquakes, and using the spectral modal analysis theory and the equivalent static load method, the calculation method of the equivalent load of casing strings under seismic action is studied. The limit state equations of various failure forms of casing strings in earthquakes are established. Based on the theory of structural reliability, the reliability model of casing strings under seismic action is built. The seismic reliability of casing strings is calculated, and the calculation results are verified by field data. The calculation results show that influence of earthquake on safety and reliability of casing strings is very serious for long service life oil and gas wells in areas where the seismic fortification intensity is high.