Abstract: A quasi plane assumption is adopted to evaluate the flexural capacity of steel-concrete box girders with corrugated steel webs.The assumption is made based on model test results and data of finite element analysis.It is validated by the theorem of minimum potential energy.Under the hypothesis,the normal stress and flexural deformation in the mid-span of steel-concrete composite box girders with corrugated steel webs agree with that of model test and 3-D finite element analysis excellently.