Objective To set up maximum bending normal stress equation for sandwich beam. Methods New methodology was proposed based on the equivalent single layer beam theory, after that the numerical examples were demonstrated according to three point flexural test. Results When the cross section of beam and the maximum loading were fixed, the maximum bending normal stress calculated by Revised single layer beam theory ( RSLBT) was increased accordingly with increasing ratio of core thickness to the whole,whereas the calculation result was a constant using the Single layer beam theory ( SLBT) . In addition, for the same sandwich construction, the difference between RSLBT and SLBT became smaller with increasing ratio of core modulus to the top. Conclusion RSLBT, which is equivalent to the Multi-layer beam theory, can provide a new method to calculate the maximum bending normal stress with suitable accuracy.