Abstract: The retreated rock-cell is a key factor of change in stability of unstable rocks in nearly-horizontal sandstone-mudstone in the part of Chongqing of the Three Gorges Reservoir. Based on parallel retreating rock-cell theory, this paper proposes the mechanical mechanism of shear failure and tension failure under retreating mudstone-cell, and stability calculation method for three kinds of unstable rocks in consideration of retreating rock-cell parameters. Pursuant to stability calculation outcome of Taibai unstable rocks in Wanzhou District, Chongqing, it is concluded as follows using the method proposed in this paper. Unstable sliding rock W12, unstable toppling rock W22 and unstable dropping rock W4 are stable. Unstable toppling rock W59 is sub-stable. Unstable dropping rock W16 is almost stable. They are identical with the field monitoring data from 2008 to 2013. Against unstable sliding rock W12, this paper discusses and concludes that the minimum principal stress decreases linearly over time. With the continuously decreased minimum principal stress, the shear failure firstly occurs to mudstone. Then the mudstone is broken with tension. The angle of rupture of mudstone unceasingly decreases with the decreased minimum principal stress. With enlarged rock-cell, stability coefficient decreases in concave curve. So enlarged rock-cell is the root cause of unstable rock failure. It exists one critical rock-cell depth. Unstable rock would break under this critical rock-cell depth. It can forecast the failure time of unstable rock.