The influence of sucrose dosage on Portland cement's setting time,strength development,chemical binding water,ion concentrations in paste liquid phase of AFt content was studied.The results indicate that sucrose accelerates the formation of AFt.When the dosage of sucrose is as low as 0.045%(by mass),the sucrose does not have much detrimental effect on the formation of Ca(OH)_2 and CS-H,the concentration of Ca~(2 ) and OH~-reduces little,the AFt has high solubility product,so that the AFt exists as gel-like products which covers the surface of clinker particles,a shell much thicker than usual is formed,thus the hydration process is restrained until the shell is broken,the sucrose acts as a retarder and does not have negative effect on 3 d strength and succedent strength development of the cement.When the dosage of the sucrose is as high as 0.48%,the sucrose affects adversely the hydration of C_2S and C_3S markedly,so the concentration of Ca~(2 ) and OH~-reduces greatly,the AFt has low solubility product,the AFt exists as well-crystallized needle-like AFt and the penetration and connection between the grains and some other hydration products are promoted,thus the setting process is accelerated,but C_2S and C_3S cannot be hydrated normally and not enough C-S-H can be supplied.The original microstructure of the paste is changed markedly,so that the strength development is baffled.It is proposed that there are two dosage threshold values as the sucrose influences the hydration of the Portland cement,the sucrose dosage of 0.01% is a retardation threshold value where sucrose can accelerate the formation of AFt,the sucrose dosage of 0.20% is a setting acceleration threshold value where sucrose can prohibit the hydration of C_3S and C_2S.