The influence of high equivalent alkali content and different alkaline metal ion (K+ and Na+) on cracking of cement-based material was investigated;the results showed that high equivalent alkali content could improve cracking sensitivity of cement-based material seriously due to its high dry shrinkage and autogenous shrinkage,in which K+ was more likely to promote shrinkage cracking.With the help of Atomic Force Microscope (AFM),the topography of C-S-H clusters, particulate interaction of C-S-H cluster of cement-based material hydration phase were characterized.K+ and Na+ were trend to limit C-S-H cluster pack each other.Based on the micromechanics parameters and molecular structure of C-S-H cluster, the important reason for improving cracking sensitivity of cement-based material was low adhesive force of C-S-H clusters caused by different alkaline metal ions.