In view of a few mechanics researches on vetiver grass applied in railway side slope protection , mechanics functions of vetiver grass were investigated through a test for vetiver grass and an analysis of the mechanics index. Vetiver grass is found to increase adhesion and internal f riction angle , enhance shear st rength and improve soil sta-bility. The test shows that only if the density of vetiver root system reaches to a certain value , shear st rength can be enhanced significantly , which implies a great role of vetiver grass in railway side slope protection.