In this study, a laboratory experiment was conducted to analyze the variation characteristics of soil thermal conductivity (λ) under different soil aggregate fractions. λ were 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 the λ of soils with a high proportion of large and medium aggregates was significantly lower than that of soils with high proportions of small aggregates; the λ of unsaturated soil and the change rate of λ with aggregate proportions of 50-25-25 or 25-50-25 was the lowest. Soil organic matter content was higher with increasing medium aggregate content. Regression analysis showed a significant linear negative correlation between the 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 with soil organic matter content. These findings provide a new idea for the study of factors affecting the λ of farmland soil.