In this study, a laboratory simulated experiment was conducted to analyze the variation characteristics of soil thermal conductivity (λ) under different size-distributions of soil aggregates. λ was measured for six proportions of large (0.25–2 mm), medium (0.53–0.25 mm), and small (< 0.053 mm) aggregates (100-0-0, 0-100-0, 0-0-100, 50-25-25, 25-50-25, and 25-25-50) by using a double-needle heat-pulse probe and time-domain reflectometer technique. Results showed that λ of dry soils with a high proportion of large and medium aggregates were significantly lower than those of soils with high proportions of small aggregates; λ of unsaturated soils and their changes with aggregate proportions of 50-25-25 or 25-50-25 were the lowest. Soil organic matter content was higher with increasing medium aggregate content. Regression analysis showed a significant linear negative correlation between λ of dry soil and the proportion of soil small aggregates, a quadratic relationship between the change of λ and the proportion of large or middle aggregates and a negative correlation between the change of λ and soil organic matter content.