Abstract: Slaughterhouse sludge, a byproduct of livestock wastewater treatment, is characterized by high organic matter content, pathogen concentrations, and significant resource recovery potential. It is often required for effective drying to reduce the moisture content before safe utilization. Fortunately, solar drying can offer a promising low-energy alternative to conventional thermal drying. However, it is still lacking in the drying behavior and thermodynamic mechanisms of slaughterhouse sludge under unsteady solar radiation. In this study, a systematic investigation was implemented to explore the thin-layer solar drying of slaughterhouse sludge. Drying experiments were conducted with sludge layer thicknesses of 5, 10, 15, and 20 mm under naturally fluctuating solar radiation, with continuous monitoring of temperature, solar intensity, and mass change. The results revealed that the drying kinetics shared a falling-rate period, with drying time extending significantly from 199 min for a 5 mm layer to 360 min for a 20 mm layer, indicating a 1.81-fold increase. Transient fluctuations in solar radiation also induced oscillations in the drying rate, highlighting the non-steady-state nature. Four thin-layer drying models—Lewis, Page, Two-term exponential, and Weibull distribution—were fitted to the experimental moisture ratio. The Weibull distribution model demonstrated that the best performance was achieved in the highest coefficient of determination (R² > 0.996), while the lowest chi-square (χ²) and root mean square error (RMSE). The parameters also showed that the scale parameter α (ranging from 170.65 to 328.09) increased with layer thickness, directly reflecting the prolonged drying time required to remove 63% of the initial moisture. The shape parameter β (1.3015–2.047) exceeded unity, indicating that drying was governed by a combination of surface evaporation and internal moisture diffusion. The positive correlation with thickness (β=0.049l+1.232, R²=0.98) demonstrated that the internal diffusion resistance dominated, as the thickness increased. The effective moisture diffusivity (Deff), solved via the Laplace transform, increased from 4.283 04×10^-8 m²/s for 5 mm to 18.737 5×10^-8 m²/s for 20 mm. This counterintuitive trend—higher diffusivity with greater thickness—was attributed to thermally driven mass transfer: the extended migration path enhanced the overall driving force for moisture removal under the non-isothermal solar drying. The activation energy (Ea), determined via an Arrhenius-type relationship, ranged from 19.21 to 38.15 kJ/mol and increased with layer thickness, indicating higher energy requirements to overcome the stronger moisture-matrix interactions in thicker layers. Thermodynamic parameters derived from the Eyring-Polanyi equation further elucidated the energy dynamics. Enthalpy change (ΔH) was positive and increased with thickness (55.04 to 62.36 kJ/mol), indicating a greater energy demand for drying. Entropy change (ΔS) was also positive and increased with thickness (123.98 to 153.80 J/(mol·K)), indicating greater system disorder and increased randomness of water molecules, as their transition from bound to free states. Gibbs free energy change (ΔG) remained positive under all conditions, thus confirming the non-spontaneous drying; However, ΔG decreased with increasing temperature, indicating that drying is less non-spontaneous and thermodynamically more favorable at higher temperatures. In summary, the Weibull distribution model can be expected to describe the non-stationary solar drying of slaughterhouse sludge, with its parameters serving as effective indicators for drying time and moisture transport. The kinetic and thermodynamic analysis can provide quantitative insights into energy requirements and process spontaneity. The finding can offer a theoretical foundation to optimize solar drying for sludge treatment. Future work can consider seasonal variations in solar radiation and integrate thermal storage or auxiliary heating for process stability and model generalization.