Unconfined compressive strength of various mixing proportions and ages of nanometer silicon and cement-stabilized soils（NCSS） are tested ,and the rules of compressive strength are got. Hydration products and microstructures of NCSS are discussed by means of XRD and SEM technology, and microstructural mechanisms of NCSS are analyzed. Results show that nanometer silicon powder added to cement-stabilized soil(CSS) can sharply improve the compressive strength of CSS. More Calcium silicate hydrates(C-S-H) and other hydration products can be produced in the process of secondary reaction of cement and water added nanometer silicon powder. X ray diffraction tests indicate the kinds and quantities of C-S-H increase with nanometer silicon contents. Strengths of NCSS are bettered by increasing jointed strength changed from edge-edge, edge-face connectios to cementation connections affected by increasing hydration products. Large pores of NCSS can be greatly decreased by adding nanometer silicon powder, and hydration products filling in the pores make NCSS more dense materials.