Abstract: The red mud-straw lightweight mortar specimens were prepared, which were based on raw materials of the Bayer red mud, slag powder, cement (P.O 42.5), as well as straw. The effect of straw content on the properties of specimens were characterized by X ray diffraction and scanning electron microscope. The results show that for cement paste hardening specimens prepared with 50% mass fraction of red mud and 50% mass fraction of the mixture of cement and slag powder, with the decrease of cement content in the mixture of cement and slag, the compressive strength of hardening specimen of red mud cementitious material paste at different ages increases at first and then decreases, when the mass ratio of cement to slag is 1:3, the compressive strength of the sample is the highest of 8.05 MPa. When straw with 10% volume fraction is added into red mud-straw lightweight mortar, the compressive strength of the specimens decrease from 10.45 MPa to 7.25 MPa. However, as the straw content continues to increase, the decrease range of compressive strength of the specimen decreases, while the flexural strength increases, and it reaches 1.9 MPa when the volume fraction of straw is 50%. With the increase of straw content, the specific weight of the specimen decreases slightly, but the thermal conductivity decreases significantly, from 0.63 W/(mK) of the specimen without straw powder to 0.40 W/(mK) of the specimen of straw with 50% volume fraction.