The estimation of grouting pressure and slurry diffusion range in grouting engineering is an urgent engineering problem to be solved, as the diffusion behavior of slurry is extremely concealed and the geological structure of underground rock and soil is complex. Taking power-law slurry as the research object, the seepage movement equation of time-varying slurry is obtained by analyzing the laminar flow movement of the slurry in the circular pipe. A cylindrical diffusion model considered the displacement of slurry-water-gas is established based on the theory of seepage in unsaturated porous media. Combined with the seepage motion equation, relevant boundary conditions and interface conditions, the power-law slurry diffusion equation is obtained, which takes into account the principal grouting parameters and mechanical parameters. At the same time, the rheological characteristics and viscosity time-varying experiment of slurry are carried out to obtain the rheological and consistency coefficient time-varying curves of slurries with different water-cement ratio, as well as corresponding function expressions. Then the numerical examples are used to analyze the mechanism of different factors affecting the slurry diffusion behavior. The above analysis and experimental results confirm the effectiveness and applicability of the research results, which can provide necessary theoretical support and technical reference for the design and construction of grouting projects in unsaturated soil areas.