Relationship between temperature and relative humidity of concrete with different mix proportions during different freezing and thawing cycles was investigated. The process can be divided into four stages, namely, freezing humidifying stage, freezing dehumidifying stage, thawing humidifying stage and thawing dehumidifying stage. During freezing humidifying stage, relative humidity increases with temperature decreasing. When relative humidity begins to decrease, freezing begins and freezing dehumidifying stage comes. During thawing humidifying stage, relative humidity increases with temperature increasing. When relative humidity begins to decrease, thawing appears and thawing dehumidifying stage comes. When water cement ratio is the same, relative humidity increases with increase of depth. When depth keeps the same, relative humidity decreases with decrease of water cement ratio, and the freezing temperature and the thawing temperature decrease. It means that the lower relative humidity is for concrete with lower water cement ratio, the higher self shrinkage and the smaller force are caused by freezing.