With the exposure time of 0.205 ms, the relationship between the object observation speed and the speckle contrast when the observable speed range includes a decorrelation area and a non-decorrelation area is investigated. The results indicate that, based on the variance trend of contrast with sampling frame and the correlation coefficient curve of time sequence speckle image, one can distinguish the speed difference between these two areas under complicated environment. Meanwhile, by choosing the suitable number of speckle image frames, one can provide a higher sensitivity for detecting the speed variance in the small speed range and the large speed range.