Abstract: Based on the research of the physical and mechanical properties of different red mud additive on red clay under various water contents, this paper show the influence and mechanism of red clay under different amount of red mud with moisture content. By scanning electron microscopy, this experiment explores the effect of soil samples on the microstructure. In the larger sense, the results show that sheer strength of soil, which is incorporatied into red mud in different ratio, increases at first and then decreases as the increase of moisture content. Compression test show that, the compressibility decreased with increasing moisture content, and the decrease amplitude increases with the growth of red mud content. From microscopic view point, the red mud additive can change the aggregation of soil particles into prothallial plate; the loose structure became denser; the reduction of pore space also makes its capacity of absorbing water is declining.