We have developed a tracking algorithm to determine the speeds of supra-arcade downflows (SADs) and set up a system to automatically track the SADs and measure some interesting parameters. By conducting an analysis on 6 flares observed by the SDO/AIA, we detected more smaller and slower SADs than prior work due to the higher spatial resolution of our observational data. The inclusion of these events with smaller and slower SADs directly results in lower medians of velocity and widths than in prior work, but the probability distributions and evolution of the parameters still show good consistency with prior work. The distributions of widths, speeds, lifetimes, and numbers of the SADs in each frame show log-normal distributions, which reveal unstable processes during solar eruptions. Also, the number of the SADs in each frame vs. time shows a rest phase of SADs, when few SADs are seen. This observation might be supportive evidence for the model that SADs originate from a plasma instability. We tentatively think that the number of SADs vs. time might related to the rate of magnetic reconnection. In addition, we discussed the possible correlations between the parameters and compared our results with a numerical simulation. It turns out the model proposed by the numerical simulation can explain our results pretty well to some extent.