Thermal conductivity is an important design parameter when recycled concrete is used for wall material. Considering the effect of four factors, including unit water consumption, water cement ratio, recycled coarse aggregate replacement ratio and recycled fine aggregate replacement ratio, an orthogonal test was conducted to study thermal conductivity and density of recycled concrete. By defining an aggregate impact factor C, the internal mechanism of thermal conductivity change was analyzed. Based on the calculation formula for the thermal conductivity of normal concrete, a modified formula for recycled concrete was proposed. The results show that the replacement ratio of recycled coarse aggregate in four factors has the greatest influence on thermal conductivity of recycled concrete. There is a significant linear relationship between thermal conductivity of recycled concrete and the value C. Calculated values from modified formula for thermal conductivity of recycled concrete agree basically with experimental values. The modified formula could be applied in the practical engineering.