Differential weathering can cause unstable rock mass in rock slope with alternating strata. The alternating strata can be sandstone and mudstone and belong to the soft and hard rocks. The rock collapse damage is a common form of geological disasters. This paper uses unstable rock mass at Puerdu as case study. The calculation formula for the compressive stress of unstable hard rock block on the soft mudstone base is derived for the slope rock mass with cavity in the soft mudstone due to differential weathering. The stress increases as the cavity depth increase. According to the slope topography, geomorphology and characteristics of geological structure of unstable rock blocks at Puerdu, two kinds of four unstable rock mechanical models due to differential weathering are established. The first type is the type of mudstone base fracturing damage and rotational damage unstable rock block whose main control fissure connectivity rate is equal to 1. The second type is the type of falling and toppling of unstable rock block whose main control fissure connectivity rate is less than 1. The rock strength theory is used to derive the quadratic parabolic Mohr strength envelope equation for sandstone at the study area. The limit equilibrium theory and Mohr strength theory is used to deduce the collapsing damage and judgment expression of four kinds of unstable rock blocks under the gravity, earthquake force and fissure water pressure. The critical collapse boundary equation is derived for the unstable rock mass at the study area. The relationships among rock cavity depth, thickness, height and the main control fissure depth are also derived. Those results provide an intuitive and reliable basis for the prediction and judgment of stability and collapse of the unstable rock mass.