Abstract: In order to study the mechanical modification effect of nano-SiO₂ on coastal cement soil at short age, unconfined compression test, splitting tensile test, pH test and SEM test of cement soil with nano-SiO₂ content (mass ratio to cement) of 0%, 1.5%, 3.0%, 4.5% and 6.0% were carried out. Mechanical tests show that nano-SiO₂ can improve the unconfined compressive strength, splitting tensile strength and elastic modulus of cement soil, but aggravate its brittleness; the compressive strength of nano-SiO₂ modified soil is about 10 times of its tensile strength. The pH test shows that nano-SiO₂ can improve the alkaline environment of cement soil. SEM micro test shows that the change law of cement soil particle morphology and pore characteristics is closely related to the strength development law. With the increase of nano-SiO₂ content, the distribution of soil particle morphology changes from loose to aggregate, and the pores decrease. In conclusion, when the nano-SiO₂ content is 4.5%, the strength improvement effect is the best, the micro pores are the least, and 4.5% is the best content. Finally, the quadratic polynomial model between mechanical strength and micro porosity of nano-SiO₂ modified cement soil is established.