Abstract：Research purposes： Biotite granites are widespread in eastern and southern China, where large quantities of high speed railways are under construction or on their research stage. Because of the great depth of completely weathering layer of granite and the special engineering geology characteristics，the slope treatment and subgrade filling become a key problem. To have a full understanding of the engineering geology characteristics of completely weathering layer of biotite granite in eastern and southern China，samples are taken in many places within the research region and laboratory tests are taken to research the mineral weathering characteristics， physical property， strength property and expansibility，which can provide certain technical support for the railway construction within this region. Research conclusions：! The mineral contents of the biotite granite in eastern and southern China are mainly quarts， feldspar and biotite，of which feldspar turns into kaolinite， illite and montmorillonite， and biotite turns into illite and composite of ferro - manganese. (2) The structure of weathering layer is loose and is mainly argillaceous cement，with the plasticity index of 10%，which belongs to the category of the silty clay. (3) The friction degree of the weathering layer of biotite granite is about 27°， but the cohesion force is less than 100 kPa， which differs from areas to areas. (4) The completely weathering layer of granite does not belong to expansive soil. (5) The research results can provide certain references to the railway survey and design passing through granites areas.