The corrosion mechanisms of chloride ion,which corroded to mortars in sands,were analyzed in this paper through microcosmic method.Based on the results,it is proposed that chlorine ion added in sands diffused gradually from the surface of sands to cement grout.The steady chloride binding ratio was about 60%.And there was an accelerated rust period before 28 days caused by dissociative chlorine ion.In the same age,the strength of mortar corroded by chloride in sands and in water was unequal.It is proved that the corrosion mechanisms of chlorine ion permeating in sands are different from others.