In view of the problems caused by expansive soil roadbed,this paper,combined with the related research of solid waste modified soil,put forward the research idea of using magnesium slag to improve expansive soil.The effects of magnesium slag on relative density,liquid plastic limit,free expansion rate,unconfined compressive strength and shear strength of expansive soil were studied by laboratory tests.The results show that the liquid limit,plasticity index and free expansion rate of the modified soil decrease with the increase of the content of magnesium slag.In addition,with the increase of mg slag content,the unconfined compressive strength and shear strength of mg slag modified soil showed a trend of first increasing and then decreasing,and reached the peak when the content of mg slag was 15%.