When evaluating the safety, reliability, and economic efficiency of casing strings in drilling operations, it is essential to define a reliable criterion of safety. According to the field statistics, when the casing tensile coefficient was in the range of 1.6-2.0, no casing failure happened in some deep and ultra-deep wells. On the other hand, the statistical data results need confirmation with an adequate scientific basis. In this paper, based on engineering experience and available research data, the authors have analyzed the types of axial load and the key factors influencing the casing tensile coefficient and proposed a new method to calculate the axial loads. Based on the reliability theory and processing quality reliability, the authors have established a method to predict and calculate the casing tensile coefficient. Based on the probability evaluation of the casing strength failure, the casing tensile coefficient is obtained. The method has been further verified by comparison with the field production data for deep wells of the Tarim Oilfield. The results show that the calculation method can reduce the acceptable casing safety value of the casing tensile coefficient to 1.5. The results have been applied and verified in the Tarim Oilfield, thus providing technical support for the development of deep oil and gas.