Abstract: In view of the water damage threat of Ordovician limestone karst in the complex structure area of the North China type coal field, the water inburst coefficient evaluation method and grouting reconstruction (reinforcement) treatment technology have not considered the impermeability evaluation performance of the ancient weathering crust in the Ordovician limestone roof. Based on the study of the physical characteristics of the ancient weathering crust on the top of the Ordovician limestone. Under the guidance of the idea of “aquifuge index method”, a aquifuge index method based on fractal theory was proposed to evaluate the impermeability property of paleo-weathering crust. The evaluation process was created and applied in the southern study area of Fengfeng mining area. The results show that the weak and weak impermeability areas of the paleo-weathering crust in the study area are distributed in strips in the middle and southwest of the mine field. The strong and strong impermeability areas are distributed in the northern and southern boundaries of the mine field, and the other areas are medium impermeability. This method can effectively reduce the number of descriptions of structural elements in the traditional evaluation methods, and can also quantitatively evaluate the complexity of fault network. It has theoretical guiding significance for the study of the impermeability of the ancient weathering crust in the North China type coalfield