Tensile cracking-sliding loess collapse is the most recurrent disaster in the expressway construction in loess area. The maximum vertical tension cracking depth in trailing edge of loess slope is obtained from the limiting equilibrium equation of soil; the constitutive relation between vertical fissure mass and the potential sliding surface mass is established by the water weakening function; the equation of balanced camber and cusp is formulated from the expression of total potential energy of the collapse

then the catastrophic model of cusp for tensile cracking-sliding loess collapse is established. The formation mechanism for tensile cracking-sliding loess collapse is explored based on the mode. After excavation

the tension crack in trailing edge of loess slope is formed

and then the fissure is filled with water. The water filling increases the shear modulus of vertical tension and decreases the shear modulus on the potential sliding surfaces. When the saturation degree of loess mass reaches a certain value

the slope system undergoes mutation and the sliding surface is formed; then the core of the soil body moves unceasingly to the free surface. Once the core of soil body sliding out of steep slope

collapse will be happen. Based on the limiting equilibrium equation and the Mohr- Coulomb's law

computing formula of the stability for cracking-sliding loess collapse is established. The research results will be of valuable reference for the design and construction of loess collapse control.