Abstract: To examine the improvement effect the content and curing time of fly ash and lime fly ash on the red clay in Guilin, the direct shear tests, oedometer tests and Scanning Electron Microscope tests are conducted. The mixture of fly ash can improve the shear strength of the red clay. After over a certain amount (18% fly ash content), the cohesion of the red clay reduces. With the prolonging of the curing time, the shear strength of the red clay with different content of fly ash increases first and then increases slowly, The shear strength of lime-fly ash red clay improves rapidly in the early stage, and increases intensely with the prolonging of the curing time. The cohesive strength of the lime-fly ash red clay increases first and then decreases with the increasing lime-fly ash content. The modulus of compressibility of the red clay increases with the mixing of fly ash and lime fly ash and increases gradually with the prolonging of the curing time. With the addition of fly ash and lime, a series of physical and chemical reactions occur in the red clay. Analysis of the micro-structure show that the pores in the soil decreases, and the structure is better than the that of the plain red-clay.