Concrete specimens of different strength grades, maximum aggregate particle sizes and specimen sizes were dried in an electro-thermal constant temperature blast oven. The specimens after drying were placed into water which was absorbed freely. The change rules of the moisture content and saturation were analyzed and the prediction formulas for concrete humidity with soak time were found. The results demonstrate that the concrete humidity exhibited a significant increasing trend with increasing soak time. Besides, the lower the strength grades, the smaller the maximum aggregate particle size, the larger the specific surface area, the water absorption rate is faster and the humidity is higher. When the soaking time is 72 h, the concrete saturation of C15, C20 and C30 specimens reaches 96. 88%, 96. 59% and 97. 70%, respectively. With the extension of soaking time, the concrete humidity increases slowly, and the water absorption process of concrete can be approximately divided into linear water absorption, nonlinear water absorption and approximate saturation stages.