Abstract：The water-resisting strata in coal seam floor will face the risk of the nonlinear instability affected by ground pressure and water pressure. The nonlinear characteristic of unstable composed water-resisting strata is studied with cusp catastrophe theory. The composed water-resisting strata can be regarded as a composed rock beam. The cusp catastrophe model of the unstable composed rock beam is established. Through the model, the nonlinear characteristic of unstable composed water-resisting strata is discussed, also the necessary and sufficient conditions are obtained. Lastly, the effect laws of ground pressure, water pressure and tunnel span on the instability of composed water-resisting strata are analyzed. The results show that with the growing of ground pressure, water pressure and tunnel span, the instability of composed water-resisting strata will be easier. It has a certain guiding significance for the prevention of floor water invasion.

Keyword：floor water invasion;composed water-resisting strata;cusp catastrophe;nonlinear characteristic;