Abstract: In many regions of our country, during design & construction of highway subbases, it often meets expansive soils which are not be the suitable soil source used directly for lime-flyash stabilized soils.In order to satisfy the requirement of engineering constructions, usually the method of lime modified soils are adopted.The mechanism analysis of expansive soils modified by lime, design of lime-flyash stabilized soils used by expansive soil, and trial road pavement and construction technology study have been conducted by lab test.The paper has concluded that mechanism of expansive soil modified by lime has embodied mainly three aspects such as dissociation of lime in water, ion exchange and flocculation & agglomeration, and a few crystal & carbonation.In construction site, it adopts a little quicklime by mixing and piling for the expansive soil so as to achieve the intent of amendments, where the mass ratio of hydrated lime, whose mass has gained by the conversion of quicklime mass, to dry soil is usually 4% and the time of amendments on site is about 7 days.Lime-fly ash stabilized soils, used by modified expansive soil, has higher early strength and after molding by compaction, and they have better water stability.The mass percent of lime and soil are the main influence factor of dry compression performances of lime-fly ash stabilized soils, made by modified expansive soil.