Abstract: The solid content in hydroxyl-terminated polybutadiene (HTPB) is usually extremely high (ϕ≥0.5), and there exists complex interactions between particles and binders as well as among particles. Its rheological properties are significantly different from those of solid-liquid systems with low and medium contents. In the present work, an inclined-plane rheometer was built to suit the specific properties of HTPB systems. Propellant substitutes with high-solid suspensions (ϕ=0.365~0.513) were prepared using high-viscosity hydroxyl-terminated polybutadiene (HTPB) as the binder and acrylate-styrene-acrylonitrile (ASA) particles as solids. A laser displacement sensor was used to measure the liquid-film thickness and surface velocity of the flows on the slope, and the explicit expressions for the power-law index n and consistency coefficient K were derived from the power-law model to establish the constitutive equation. The results show that n falls from 0.893 to 0.432 and K rises from 153.6 Pa·sⁿ to 670.9 Pa·sⁿ as ϕ increases from 0.365 to 0.513. These phenomena reflect strong shear-thinning behavior, and these model parameters, i.e. n and K, exhibit exponential dependence on solid content ϕ. Empirical correlations for n and K as functions of ϕ are now available with a prediction error below 10%. The method provides a simple and reliable means of characterizing high-solid HTPB propellants and delivers the essential physical property data required for equipment design and process optimization.