Abstract: It is an important precondition to study the process of grouting fluid diffusion and the law of reducing seep- age in coal rock fractures for improving the holes sealing quality and the gas extraction efficiency. Firstly,by analyzing the internal mass variation of fractured coal samples,the mass conservation equation and continuity equation of grou- ting fluid sealing fractures are derived. The permeability variation model is derived based on the influence of grouting fluid on coal void structure,building the variable mass seepage model for plugging fractures and reducing seepage. Sec- ondly,the grouting experiment system is used to conduct grouting experi-ments on fractured coal samples, and CT scanning observations and permeability experiments are respectively performed for coal samples before and after grou- ting. Finally,based on the theoretical model,the numerical simulation initial values and boundary conditions are deter- mined according to the experimental conditions. The numerical simulation study of the cracking law shows that the established variable mass seepage model based on fractured coal body grouting better reflects the influence of grouting fluid particle deposition process on coal fracture structure and permeability. The grouting fluid diffusion process and the law of reducing seepage for fractured coal samples are studied by using numerical simulation. The study results in- dicate that the variable mass seepage model for grouting fluid in fractured coal can generally reflect the influence of particles sediment on the fractured coal and permeability,and the numerical simulation result is in good agreement with the experimental results,which verifies the correctness of the model. Under the action of seepage and diffusion,the grouting fluid particles in the coal body cracks gradually diffuse and deposit,and the fractures are continuously filled, resulting in the continuous decrease of the permeability of the fractured coal. The research results provide a new idea for the quantitative analysis of the migration law of grouting fluid particles and strengthening the sealing effect of gas extraction boreholes.