When detecting a small slow-moving object in a low SNR multi-frame image, characteristics of size, shape and vein can not be used to distinguish the object from untrue ones formed by noises. Based on the slow-moving characteristics of small objects and the stochastic of background noises, the method of frame integral can be employed first to detect the object, followed by region growing to extract group pixels of it,and further the effects of the number of integral frames, the moving speed of the centroid and the rules of image processing on the single frame detecting probability and extractive results are analyzed and validated. By comparing the effects of one object moving at two different centroid rates, it is demonstrated that the method of frame integral can predict traces of unslow-moving objects, but the opportunity of skipping useful single frame is high.