As the classical image segmentation algorithm, Ostu threshold algorithm has been widely used in image processing. The using of the algorithm which was developed on the basis of two-dimensional, high-dimensional threshold algorithm was restricted because of its complexity. But they consider both the gray value of a pixel and the average gray value of its neighborhood, thus is more robust to noise. By constructing look-up table recursively, its fast algorithm reduces its complexity from four-dimensional to two-dimensional. Algorithm based on the decomposition threshold selection, using threshold of two one-dimensional Ostu algorithms to replace it of two-dimensional Ostu, the method is the same as two-dimensional Ostu algorithm in segmentation threshold, and the algorithm's time complexity can be reduced to one-dimensional. In the paper, the algorithm was improved, one-dimensional threshold average decomposition for two-dimensional Ostu algorithm was proposed. It can be confirmed by a large of number experiments, The algorithm not only maintained advantages of less time and smaller space of calculating the threshold but also better segmentation results can be found from experiments.