The invention discloses a method for improving acid soil by using montmorillonite. According to the method, montmorillonite powder is applied to the acid soil, the montmorillonite powder and the acid soil are mixed evenly to form evenly mixed soil, and the evenly mixed soil is left to stand in an outdoor environment until the pH value of the evenly mixed soil is stable. The method has the advantages that the soil modification quality is stable, secondary pollution is avoided and the like, and the acid soil can be improved effectively to be suitable for cultivation.