The improvement effect of modified coastal cement soil by nano-clay under freeze-thaw cycles was evaluated from the view of soil microstructure. The change law of soil microstructure was analyzed by SEM micro test, and the correlation between the quantification of soil microstructure and macro-mechanical characteristics was explored. The microstructure characteristics and quantitative parameters of coastal cement soil and nano coastal cement soil under freezing and thawing cycles of 0,1,5,7,9 and 11 were obtained by scanning electron microscope and Image Pro Plus software. The results show that:(1) Under the condition of no freeze-thaw cycle, the effect of nano-clay on improving coastal cement soil is obvious. (2) In freeze-thaw environment, the effect of nano-clay on coastal cement is good at the early stage of freeze-thaw cycle, but the effect decreases with the increase of freeze-thaw cycles. In 0-5 cycles of freeze-thaw cycles, the pore area of nano coastal cement soil decreased by 14.3% on average. While in 7-11 cycles of freeze-thaw cycles, the pore area decreased by 4.3% on average. According to the distribution of the probability density function, the nano-clay inhibited the development of the super-macropores in coastal cement soil. (3) The compressibility of nano-sized soil is sensitive to the change of particle area, and the number of particles and the coefficient of inhomogeneity have little effect on the compressibility. The results indicate that the nano-clay can improve effectively the freeze-thaw resistance of the coastal cement soil in a short time.