Abstract: The concrete stress caused by temperature variation and shrinkage is related with the stiffness of the member.When cracks occur on a reinforced concrete beam due to temperature and shrinkage,the axial stiffness will drop,and so will do the internal force.Research into this field is still qualitative so far.The development process of concrete stress under temperature decrease and concrete shrinkage was simulated,and iteration method was used to solve the reduction of internal force and axial stiffness.And some reasonable results were also got,which were worth consulting.