Abstract: Fractured rock mass is one of the main water inrush channels. Groundwater exhibits nonlinear seepage characteristics in fractured rocks when water inrush occurs. However, the research on the risk of water inrush from the fractured rocks mainly focuses on qualitative analysis due to the complexity of the internal structure of the fractured rocks, which is difficult to be quantitatively evaluated. In order to find a quantitative evaluation method for the risk of water inrush from fractured rocks, the experiments of nonlinear seepage in fractured rocks were carried out to investigate the changes in the internal structural parameters of fractured rocks and the parameters of nonlinear seepage in fractured rocks. A nonlinear seepage mathematical model was derived based on the nonlinear seepage theory and fractal theory of porous media. The relationship between the structural characteristic parameters of the fractured rocks, nonlinear seepage characteristic parameters, and the classification of the water inrush risk was established. An evaluation criterion of water inrush based on the critical pressure gradient of nonlinear flow was established. The results indicate that: the critical pressure gradient of nonlinear flow can be used as a quantitative indicator to evaluate the risk of water inrush from the fractured rocks, which considers both the internal structural parameters of the fractured rocks and the nonlinear flow state parameters of groundwater; The evaluation criterion of water inrush based on the critical pressure gradient of nonlinear flow considers the total critical water pressure that applied to all rock layers including the fractured area and intact rock layers, compared to the traditional method of determining the risk of water inrush, which only considers the water resistance ability of intact rock layers. Also, the evaluation criterion considers the water resistance ability of all rock layers. It is the first attempt to apply nonlinear seepage state parameters to the study of water inrush hazard quantification methods. It is also a water inrush hazard quantification method that couples the internal structural parameters of fractured rocks with the flow state parameters of groundwater.