Maximum bending normal stress is one of the important parameters to measure flexural property of sandwich beam. Therefore，new methodology to calculate the stress is proposed based on the equivalent single beam theory，after that the numerical examples are demonstrated according to three point flexure test. The results showed that revised single beam theory (RSBT) are equivalent to multi-layer theory (MLT). When cross section of beam and maximum loading are fixed，the maximum bending normal stress calculated by RSBT is increased accordingly with increasing ratio of core thickness to the whole，but the result is constant for single beam theory (SBT). RSBT is set up to provide a new method for sandwich construction application in engineering industry.