The functional equations of internal humidity and saturation fraction in concrete induced by self-desiccation effect are presented based on theoretical studies and tests on the relative humidity change in concrete. Digital humidity sensors are used to measure the internal relative humidity change against concrete age under sealed curing. The tests manifest that the relative humidity in concrete with low water-cement ratio decreases rapidly in the early-age, which indicates that the cracking probability of concrete with low water-cement ratio is significantly increased. There is a quite small relative error between the test values of relative humidity and the calculated values from the model proposed in the paper.