This paper at first reports an experimental work to test the thermal conductivity of formulated sandy soil specimens of different porosities and water contents. Both needle probe and hot-plate methods were conducted and compared. It has been confirmed that the needle probe method has a better accuracy for the measurement of unsaturated soil thermal conductivity, and that the pore size distribution plays an important role on unsaturated soil thermal conductivity. Secondly, it gives out an extensive review on the modelling work, and investigates two major types of empirical models in detail. Two generalized formulas have been suggested to mathematically characterise the two types of models and tested using the experimental data. Finally, in terms of the intrinsic relation to the pore size distribution and the similarity between the thermal conductivity and water retention characteristics of unsaturated soils, a soil water retention model has been suggested and tested to describe unsaturated soil thermal conductivity.