Soil-bentonite mixtures are often used to maintain the stability of the deep mixed pile method for the double round milling wall (Cutter Soil Mixing, CSM). The effect of bentonite content on the unconfined compressive strength and permeability of soil-bentonite mixtures in CSM when the local soil was sand and silt. The properties including the unconfined compressive strength and permeability was measured, and microstructure was investigated using MIP. The relationship between the change of pore ratio and the unconfined compressive strength and permeability coefficient of cement soil samples was investigated. The results show that the expansion of the bentonite and chemical reaction with cement hydration products can change the pore distribution of soil-bentonite mixtures; The variation of bentonite content can significantly reduce the permeability coefficient and improve the unconfined compressive strength; The increase of unconfined compressive strength of sand samples is larger than that of silt samples;The ratio of compressive strength to porosity and cement content is approximately a power function relationship; Bentonite can effectively fill the pores, and at the same time, it reacts with cement hydration products to change the pore distribution in cement soil; A certain amount of bentonite is effective in improving the strength of the cement soil sample, but the marginal effect of improving the dosage is not obvious. The optimum content of bentonite in silt is 5% and in sand is 2.5%~5%.