The law of expansive power of sandstone on the upper plank of coal bed in Guhanshan coal mine in Jiaozuo under high temperature is studied; and both the ultrasonic wave parameters and the porosity are also studied in order to find out the mechanism that causes the expansive power of sandstone under high temperature. The results show that the expansive power of the sandstone will become great gradmally when the temperature increases; and expansive power of the sandstone is small when the temperature is under 300 ℃; but it will become greater when the temperature is greater than 300 ℃. The study of the ultrasonic wave parameters and the porosity also indicates that the expansive power of sandstone will appear.