Abstract: Variational principle is usually fit for elastic analysis of shear lag effect in box girder. In this paper, based on transformed-section method and dismemberment method, shear lag coefficient formulas of continuous reinforced concrete box girder under uniform load is derived by means of variational principle, in which concrete cracking is taken into account. The result is compared with test data and that adopted by design code. Analysis showed that concrete cracking would change distribution law of shear lag effect, and its effect is small on the effective width coefficient of flange at mid-span, its effect is big on that at mid-supported section. The method adopted in this paper is definite in mechanical concept, and it is the extension of elastic analysis, it can be widely used in shear lag effect analysis of cracked continuous box girder.