To reveal the differences of the design approaches provided by Standard for Design of Steel Structures (GB 50017—2017) and Standard for Design of High Strength Steel Structures (JGJ/T 483—2020), and to propose more accurate formula for determining the ultimate moment of Q460 steel beam failed by global buckling, a general index-form formula for the global buckling capacity was proposed by summarizing the literatures. Based on the design provision of GB 50017—2017, the stability factor in unified form was proposed, also with the corresponding "stability factor and slenderness ratio" curve. The "stability factor and slenderness ratio" curve of ideal beam was also proposed. By comparing those "stability factor and slenderness ratio" curves, their characteristics were concluded respectively. The problems of index-form formula in current design standards were also pointed out. Based on the analysis on the benchmark moment, a new index-form formula for the global buckling capacity of beams with Q460 steel was proposed. The problem that the slenderness ratio could not be determined in JGJ/T 483—2020 was solved. Test results for fourteen specimens in literatures were used to assessing the precision of the proposed formula. The results shown that, the propose formula for determining the ultimate moment of Q460 steel beam failed by global buckling, is more accurate than the design approaches provided by both GB 50017—2017 and JGJ/T 483—2020.