Water exchange and migration of superabsorbent polymers (SAP) in cement paste is a very complicated process and governs the effectiveness of internal curing in concrete. The early state of SAP in real pore fluids was investigated under different water-cement ratios and hydration time. It is found out water absorption of SAP is not a constant value, but varies depending on these two factors. The absorption and desorption behavior of SAP is demonstrated in the shifted hydration peak location and the water-rich region around the SAP gels. The water exchange between SAP and cement pastes is quantified by the measured water content with time, from which a characteristic “two-stage desorption” driven by osmotic pressure and humidity gradient, respectively, is identified. The humidity-controlled desorption that is beneficial for internal curing is calculated. It is indicated the internal curing efficiency could be improved by shortening the osmotic stage and prolonging the humidity stage. Display omitted