Research conclusions: (1) Black cotton soil has engineering properties like medium-strong expansion, medium-high compression, etc. (2) It proposes the following subgrade treatment measures: strengthening the subgrade drainage system; when cut slope height , 3 m，using shrub and grass for slope protection; when cut slope height>3m, using skeletons protective slope to protect side slope; if the slope is high, set retaining walls; cutting subgrade bottom layer with weak-medium expansive soil shall have replacement no less than 0.5m，cutting subgrade bottom layer with medium-strong expansive soil shall have replacement no less than 1m.(3)It provides guidance and reference for survey and design work of railways, highways, light rail transit construction in Ethiopia.