Abstract: Seismic reliability analysis of casings string was brought forward in pursuit of safety of long life oil gas wells. The randomness of earthquake was described, and the regularities and parameters of distribution of earthquake intensity and the acceleration of ground motion were obtained. In addition, the regularities and parameters of distribution of casing’ outside diameter, wall thickness, Young’ modulus, and Poisson’s ratio were clarified by means of description of the randomness of casing’s geometric dimensions and mechanical properties. The mechanical model of the casing string under the earthquake action was founded. The study of calculation method of equivalent stress of casing string under the earthquake action was completed in virtue of the spectral modal analysis theory and the method of equivalent static load. The reliability model of casing string under the earthquake action was established by dint of structure reliability theory. The seismic reliability of practical casing string was calculated by means of Latin Hypercube stochastic simulation method based on Monte-Carlo method. The calculation result shows that casing string’s security index follows the minimum extreme value type I distribution, and the influence of earthquake on casing string is always very low. However, the influences of earthquake on casing string are very serious in long life oil gas wells in areas where the seismic fortification intensity is higher.