After the operation of the Three Gorges cascade reservoirs (TGP), the mid-channel bars (MCBs) downstream of the dam were generally scoured and shrank, which directly affects the river regime in ana-branching reaches and the stability of the navigation channel boundaries. The morphological evolution processes and mechanisms of MCBs in the Zhijiang (ZJ) and Shashi (SS) sub-reaches of the Upper Jingjiang Reach were analyzed, using the remote sensing images, the hydrological data and the measured riverbed compositions. The results indicate that: (i) During the post-TGP period (2003-2019), the sandy MCBs in the SS subreach shrank more significantly than the one in the ZJ subreach. The total exposed area of the former decreased by 31%, while the latter decreased by 24%; (ii) Guanzhou (GZ) and Jinchengzhou (JCZ) were taken as the representative sandy-gravel and sandy MCBs, respectively. During the post-TGP period, the head zone of the GZ MCB was relatively stable, and the reduced area was mainly in the bar-tail zone due to the illegal sand mining, while the JCZ MCB had a more obvious erosion trend; (iii) The bed-material composition had an important influence on the erosion degree of MCBs. The GZ MCB had a stronger anti-erosion capacity than the JCZ MCB, which was related to the less amount and duration of bed material that can reach the incipient condition in a hydrological year. Moreover, different empirical relationships were developed between the exposed area of MCB, fluvial scour intensity, and relative water depth. It is found that comprehensive consideration of the effects of flow-sediment condition and bed-material composition adjustment can better explain the recent variations in the exposed areas of MCBs.