Abstract: Downburst is a local severe weather phenomenon. It not only causes great harm to flight safety, but also serious disasters to the ground objects. According to the downburst's characteristic that when downburst happens, it's reflectivity factor core of its matrix drops rapidly. This article is based on the data of the radar scanning, defines the concept of "unit area equivalent potential energy" for judging downburst's occurrence and development and, combining the fact of thunderstorm's velocity field convergence in the mid-level during it's occurrence, then gives the downburst's outflow strength formula. By analyzing the measured data, the calculation results shows: (1) Changes in the unit area equivalent potential value accurately reflect the strong center height variation of thunderstorm. (2) The equivalent potential energy difference of the adjacent volume scan reflects the quantity of thunderstorm's energy released: before the downburst occurs, the potential difference of the 1-2 volumes scan reaches maximum, which reflects outbreak of downburst. By setting the release quantity of the unit area equivalent potential energy to 30% one can early warn the happening of downburst. (3) The release of the unit area equivalent potential energy can early make the warning of downburst occurrence in 6-12 minutes in advance, and through the flow strength formula one can quantify the value of the ground winds generated by the downburst. The error of early warning wind speed is determined by the distance from thunderstorms to radar, the closer it is the smaller the error of wind speed is, and vice versa. And,(4) the six downburst warn results show that the index can accurately warn the occurrence of downburst, while the flow strength is greatly influenced by thunderstorm identification parameters, with the error being larger in the case of the distance farther.