Using clean water and sulfate solution as the wetting medium，the internal relative humidity in concrete under dry wet cycles was measured. The test results show that at the beginning of concrete placement, the variation law of the internal relative humidity in concrete with age can be described by a water vapor saturated stage with 100% relative humidity followed by a stage that relative humidity gradually decreases and significant humidity gradient along the height can be found. Under dry wet cycles, the changes of the relative humidity occur just within a certain depth from the concrete surface. This depth is commonly known as the influencing depth. Under the same dry wet cycles, different influencing depths were observed for concretes with different strength. The lower the water to cement ratio and/or the higher the concrete strength, the shorter the influencing depth. The influencing depth is larger when sulfate solution is used as the wetting medium than that when the clean water is used.