After the operation of the Three Gorges Project, the bed material in the Jingjiang Reach has suffered from adjustment in different degrees, and the bed material continuously exchanged with bed load and suspended load, which has a significant influence on the bed evolution and non-equilibrium sediment transport in local subreaches. In this study, the exchange of bed material and the movement of sand waves are firstly analyzed and improved, with the hiding-exposure effect and three-state transition probability of nonuniform sediment being considered; Finally, the equilibrium equation for calculating the grain size distribution in sand-bed reaches is proposed based on the threshold probability. These calculated results show that: (1) During 2009 and 2014, the annual process of median grain size at Shashi Station firstly increased and then decreased, while the median grain size at Jianli Station increased after decreasing; The composition of surface bed material in the Jingjiang Reach showed an upward increasing trend, and the coarsening degree was about 6.9%-9.3%; (2) The percentage of fine sand in bed material remained unchanged, and the medium sand decreased while the coarse sand increased from 2009 to 2014 at Shashi Station (the variation was about ±11.4%); There was a slight change in the composition of bed material at Jianli Station; (3) The transition probability from bed material to bed load increased with an increase of grain size (d), while the probability to suspended load decreased with it, and the probability from bed load and suspended load to bed material increased with an increase of d; The main form of sediment transport was the suspended load when riverbed scouring occurred (the probability was 81%-87%), and the supply of bed material mainly came from bed load when riverbed deposition occurred (the probability was 8%-12%). The probabilistic model developed in this study shows reasonable agreement against the field observations at the Shashi and Jianli Stations from 2009 to 2014, and it can also provide a theoretical basis for further studies on the mechanism of non-uniform suspended sediment recovery in the reach downstream of the Three Gorges Dam.