Influences of pre soaked light weight aggregate(PSLWA) volume fraction and water binder ratio on early age interior relative humidity(IRH) distribution and development of high , moderate and low strength concrete slabs with casting surface exposed to indoor environment were investigated. Results show that with the increase volume fraction of PSLWA, the early age globle IRH of concrete slabs are gradually increased, the duration of humidity saturated stage is noticeably prolonged, the IRH reduction process is delayed and mitigated, the IRH non linear gradients along the slab thickness are reduced, and the ratio of IRH saturated zone is increased. As the internal curing water reaches about 60kg/m3, the globle IRH of high and moderate strength concrete slabs stays above 92% within 28 days. The lower the matrix water binder ratio, the stronger the positive internal curing effect of PSLWA, and the thinner the zone affected by surface drying. However, internal curing by PSLWA could not eliminate the surface drying effect of high , moderate and low strength concrete slabs under indoor dry environment.