The transformation rule of different forms of water of internal curing pavement concrete was researched at different ages, and the influences of SAP particle size and dosage on the relative humidity, shrinkage performance and flexural strength of pavement concrete were explored. The influence mechanism of internal curing on flexural strength was revealed. The results show that SAP with smaller particle size could release the water gradually for a longer period of time. The shrinkage reduction rate of internal curing pavement concrete on 3，7d was 6885% and 7398%, respectively. Internal curing technology could effectively reduce the vertical humidity gradient of pavement concrete and improve the homogeneity of material. SAP with smaller particle size plays a good hydration filling role on its residual holes, which could enhance the bonding strength of aggregate cement matrix interfacial transition zone(ITZ).