A theory of seepage instability was used to estimate the harmfulness of water-inrush from a coal seam floor in a particular coal mine of the Mining Group,Xuzhou. Based on the stratum column chart in this coal mine,the distribu-tion of stress in mining floors when the long-wall mining was respectively pushed along to 100 m and to 150 m was simulated by using the numerical software (RFPA2D). The permeability parameters of the coal seam floor are described given the relationship between permeability parameters. Strain and the water-inrush-indices were calculated. The wa-ter-inrush-index was 67.2% when the working face was pushed to 100 m,showing that water-inrush is possible and it was 1630% when the working face was pushed to 150 m,showing that water-inrush is quite probable. The results show that as long-wall mining is pushed along,the failure zone is enlarged,the strain increased,and fissures developed cor-respondingly,resulting in the formation of water-inrush channels. Accompanied by the failure of the strata,the perme-ability increased exponentially. In contrast,the non-Darcy flow β factor and the acceleration coefficient decreased ex-ponentially,while the increase in the water-inrush-index was nearly exponential and the harmfulness of water-inrush in the coal mine increased accordingly. A theory of seepage instability was used to estimate the harmfulness of water-inrush from a coal seam floor in a particular coal mine of the Mining Group, Xuzhou. Based on the stratum column chart in this coal mine, the distribu-tion of stress in mining floors when the long-wall mining was changed by about 100 m and to 150 m was simulated by using the numerical software (RFPA2D). The permeability parameters of the coal seam floor are described given the relationship between permeability parameters. Strain and the water-inrush-indices were calculated. The wa-ter-inrush-index was 67.2% when the working face was pushed to 100 m, showing that water-inrush is possible and it was 1630% when the working face was pushed to 150 m , showing that water-inrush is quite probable. The results show that as long-wall mining is pushed along, the failure zone is enlarged, the strain increased, and fissures developed cor-respondingly, resulting in the formation of water-inrush channels. Accompanied by the failure of the strata, the perme-ability increased exponentially. The non-Darcy flow β factor and the acceleration coefficient decreased ex-ponentially, while the increase in the water-inrush-index was nearly exponential and the harmfulness of water -inrush in the coal mine more accordingly.