Coal gasification slag is one of the major solid wastes produced by modern coal chemical industry in recent years. In this paper, the setting time and compressive strength of ordinary Portland cement with different contents of coal gasfication slag were studied. The mechanism of the action of the coal gasfication slag in ordinary Portland cement was discussed and analyzed by XRD, FT-IR and SEM techniques. The results show that, in cement stone, the unreacted coal gasification slag mainly exists in forms of aggregate state. Low-content gasification slag (10%) plays the role of nucleation in cement paste, which is favorable to the hydration reaction of cement, resulting in the increasing number of hydration products, the shortening of the setting time of cement paste and the improving of the compressive strength of cement stone. When the content of gasification slag is more than 30%, the amount of the hydration products of cement decreases and the structure of cement stone is loose. With the increase of the content of the coal gasfication slag, the setting time of cement paste is significantly prolonged, and the compressive strength of cement stone decreases markedly.