Background extraction is one of the most important steps to get the obtain target by using background difference. The question of the traditional background extraction is to deal with the large amount of data. In order to deal with this problem, this paper proposes a modified background extraction algorithm, based on the traditional background extraction algorithm. It is the principle of the algorithm that the grey value of adjacent images and the same position belongs to the same smooth range in a period of time. According to the principle, we can deal with small amount of data by deal with many groups. The experimental shows that this approach can be getting a better effect, and using less memory.