Limestone calcined clay cement （LC3） mortar was prepared by using light ceramsite sand， cement， metakaolin and limestone powder. The influences of ceramsite sand content and prewetting state on the dry density， mechanical properties， impermeability and resistance to chloride ion erosion of LC3 mortar were investigated and the strength formation and microstructure mechanism were analyzed. The results show that the dry density and compressive strength of LC3 mortar decrease gradually with the increases of the mass ratio of ceramic sand to binder. The cumulative water absorption of LC3 mortar increases， while its chloride ion penetration resistance increases. However， compared with LC3 mortar without prewet ceramic sand， the 28 d compressive strength of samples mixed with prewet ceramic sand increase by 3.4%-10.8%， the 28 d electric flux decrease by 59.0%-80.0%. In addition， the cumulative water absorption decreases more significantly. For the LC3 system， the nucleation of limestone powder promotes the early hydration of cement and the pozzolanic reaction of calcined clay is contributed to refining pore structure. Meanwhile， the internal humidity of the matrix and the hydration of cement is promoted because of internal curing effect of the prewet ceramic sand， which improves microstructure of the prewet ceramic sand LC3 mortar more dense， and thus its mechanical properties， impermeability and resistance to chloride ion erosion are enhanced.