The formative mechanism of weak interbeds within the discharge material stack field base loess of the Antaibao Opencut Coal Mine is discussed in this paper, based on studies of clay minerals, microtextures and microtexture evolution under different load pressures. The research demonstrates that the weak interbeds are a kind of evolution ones formed by microtexture changes of the base loess under the load pressure of the overlying discharge materials, and constitute the controlling factor of the discharge material stack field landslides. They do not exist and cannot be detected during the time of designing a new discharge field. But their influence on the stability of the discharge field slope must be considered in order to avoid the danger of landslides.