A new measure for canal slope of expansive soil which uses geogrid to strengthen expansive soil and expansive soil as backfill is studied.Base on the consideration of some key factors such as the interaction between geogrid and expansive soil, expansion which is caused by absorbing water and softening characteristics,a new way of calculation is presented for canal slope of expansive soil which is strengthened by geogrid,and this method is verified by FLAC3d.The result shows that geogrid can obviously restri...