By using Hellinger-Reissner's calculus of variations,this paper presents the usual method with which the section strain distribution state of reinforced concrete member can be derived from the basic balance equation.This method is suitable for any difinite form of strain distribution of the compressional zone of concrete.It is made clear by calculation that if the compression zone of concrete has linear strain distribution in the deformation process,the strain of tension bar and the distribution mentioned above can satisfy the assumption of plane section multaneously.