Iron tailings are the remains of solid waste after iron ore concentrates are selected by mineral processing technology. For a long time, iron tailings are disposed through accumulation and burial, which not only occupy large space but also increase the cost of treatment. Ground source heat pump (GSHP) system is a kind of environmental-friendly, energy-saving system by using geothermal energy. This is getting more and more wide use in China. The main problem of using GSHP is the low thermal property of backfill materials in the loess region of Northwest China, which affects the thermal efficiency of GSHP system. Improving the thermal conductivity of backfill materials is necessary and urgent for energy conservation. The authors hypothesized that thermal efficiency increases as the rate of tailings addition increases. To test this hypothesis, the authors applied iron tailings to the backfill material. Experiments were conducted to measure the thermal conductivity of the loess and iron tailings, determined under the conditions of different dry density, water content, and mixing ratio. Then, the changes of thermal conductivity were analyzed as tailings' mix proportion varies. The optimum mix ratio was determined and the reason was discussed. The results showed that the thermal conductivity of backfill material is a formula of saturation and rate of tailings addition. A series of functional equation was given in the end. The calculated value of the formulae matched well with the experimental data. The research concluded that the iron tailings as backfill material of GSHP is feasible and the result can be used as a reference for related engineering research. (C) 2018 Elsevier B.V. All rights reserved.