The conception of equivalent stiffness is used to simulate structural damage severity. Using this approach

an analytical model

which includes three damage parameters（damage location

damage severity

and the length of damage）

is developed for fixed-fixed beam. Solutions for the bending moment along the fixed-fixed beam under dead load

for both the undamaged state and damaged state

are obtained by solving the equations which are deduced from the theories of plane bending and approximately differential equation of flexural. The change rules of bending moment for damaged fixed-fixed beam are summarized from comparing the numerical values. The purpose of this paper is to lay groundwork for future research about structural health monitoring and damage identification.