The elastic overall stability factor of singly symmetric I-section steel beams in Standard for Design of Steel Structures(GB 50017—2017) is an important factor for calculating the overall stability bearing capacity of steel beams, but the factor is obtained through a series of simplifications, which does not meet the current requirements for refined calculation. Formula for warped moment of inertia in summation form was proposed and other geometric parameters in summation form were then given. Based on the first yield moment criterion, formula for elastic overall stability factor was defined by employing general expression of critical moment and elastic ultimate moment, and precise formula of elastic overall stability factor of steel beams with singly symmetric I-section was derived. The problems in Standard for Design of Steel Structures (GB 50017—2017) were revealed on the basis of the precise formula of elastic overall stability factor and geometric parameters in summation form. Comparisons were made for verifying the correctness of the precise formula of elastic overall stability factor by employing the results in literatures. The results show that the problems of overall stability factor in Standard for Design of Steel Structures(GB 50017—2017), are oversimplification of formula of geometric parameters, limited application range of equivalent moment factor, and the wrong expression of steel grade correction factor. The precise formula for the overall elastic stability coefficient of a single axis symmetrical I-shaped section steel beam is not only widely applicable, but also has very high calculation accuracy.