Abstract: The geological genesis of red clay is the fundamental source of its special engineering characteristics. Based on analysis of Guangxi karst landscapes and distribution characteristics of the red clay, this paper chose 4 kinds of red clay in Guangxi karst area as the research objects to make an in-depth analysis of the engineering geological characteristics and genesis of red clay in karst area of Guangxi by field sampling, profile feature description, laboratory soil test and microscopic experiment. The research shows that:(1)The typical hot-humid climate and well-developed karst environment in Guangxi are beneficial to the formation of red clay.(2)Guangxi red clay has the general characteristics of red clay, such as the mineral component with kaolinte, illite, montmorillonite and thedouble pore structure with aggregate in the micro pore and particle mass pore.(3)It is the characteristics of Guangxi red clay from the surface to the bed rock that void ration, water content liquid limit, plasticity index, free swell and shrinkage increase gradually, while the density and dry density becomes smaller, and it shows obvious stratification.(4)Considering the geological genesis, the red clays from Guilin, Liuzhou and Wuming are residual red clays, and Laibin red clay is an alluvial red clay.