An improved polygons merging algorithm (hereafter “PM” algorithm) was developed for the operational test bed of classified severe weather probability forecasting, based on Weiler Atherton algorithm. In this algorithm, the input points can be non directional. It is a simple and efficient polygons merging algorithm that meets the basic requirements of the meteorological operation and can also be used for polygons clipping. During the period from April to September 2013, it was tested on the test bed platform and proved to be exact, efficient and stable. In addition, it can be used in some other relevant fields of weather forecast operation, such as disaster weather forecasting, environmental weather forecasting, special weather forecasting and soon.