In order to solve the problem that the internal force of statically indeterminate prestressed and reinforced concrete bar system structures is difficut to determine when the structures come to the non-elastic state, a simple analysis method of experimental structures' internal force-sectioned dropping rigidity method is brought forward.In this method, according to the loading state of bars, the bars are divided into limited segments by the level of rigidity dropping, and the relative level of each segment is enacted; then the dropping modulus of each segment rigidity will be found by debugging time after time, and the checking point is the key distortion which was gained from the experiment; it is considered that the internal force which is calculated by elasticity analysis at the dropped rigidity can reflect the real internal force of the structures.It is basically proved that the method, which is brought forward in this paper, is effective by comparing its results with those from the experiment of a large size prestressed frame of two spans and single floor, of which the internal force redistribution is considered.