In order to investigate the plastic design method of indeterminate encased steel box concrete beam (ESBCB),4 two-span continuous ESBCBs are tested,and then the distribution and growth of crack,deformation,failure characteristics and redistribution of plastic internal force are obtained.The calculation formula of the length of equivalent plastic hinge of ESBCBs is presented.A design method for moment modification is put forward,in which the object of moment modification is the difference between the elastic calculated moment of critical section at intermediate support and the moment subjected to the steel box when the composite beam reaching the limit state of bending capacity,and the variable is the relative plastic rotation.