Digital mask technology is a method with good prospect that manufacture diffractive optical element.While diffractive optical element is made,the number of gray level that can be used will be less than 256 due to sensitive nonlinearity of photosensitive material.Even if 256 gray grades can be used entirely,fine control that obtains general processing demand will not be realized.In the paper,two methods of gray subdivision and expansion are put forward.They are assembled modulation by many SLMs and color-equivalent-gray technology.It is analyzed theoretically that two methods both can realize gray subdivision,which can result in elaborate control of light exposure.