Based on the equilibrium of static forces and coordination of deformations, with considering the effects of bond strength variation and crack width development at different stress stage of reinforcement in tension cracks, the average stress strain relationship of unanchored zone of RC structure under bond and slip action has been derived. The typical test on reinforced concrete beams was simulated with average constitutional relationship and the result was compared with that of real test. The analytical results show that the model using average constitutional relationship with considering bond and slip in unanchored zone of RC structure can give a result close to that of test and simplify the nonlinear analysis.