Abstract: Taking fully-mechanized mining face with large mining height under the huge thick main roof as engineering background and using the ways of field measurement and theoretical analysis, the breaking model of huge thick main roof was established in the process of periodic weighting, the layer thickness and the carrying capacity of gangue was determined. The results show that the working resistance of hydraulic support was small during normal mining period, but the working resistance rapidly increased once it encountered periodic weighting, at the same time the dynamic load coefficient was larger. The lower layer formated the structure which is called “inverted trapezoidal combination of cantilever beam” and the upper layer developed a stable balanced structure under the support of coal gangue when the huge thick main roof delaminated after mining. The hydraulic support only assumed static load of “inverted trapezoidal combination of cantilever beam” during normal mining period. With the extension of the upper layer rock block, the carrying capacity of gangue was inadequate and instability of balanced structure caused periodic weighting.