Based on high spectral reflectivity features of Blue-green Algal Bloom(BAB) in Lake Dianchi, by MODIS satellite re-mote sensing data, RGB;621 combined false color synthesis and Normalized Difference Vegetation Index(NDVI)method, we found that green region in false color synthesis image and the region with NDVI value>—0. 1 were just the BAB area. When—0.1,NDVI≤0. 2, slight BAB occurred, and bloom coverage in a pixel reaches 0—30%; when 0. 20.4,and bloom coverage in a pixel can be 81%—100%. From June to September is the key period for BAB breaking-out in Lake Dianchi, and the key factors to affect BAB breaking-out maybe sunshine and wind velocity. During this period, if the sunshine can last for only 4—5 hours and the wind velocity≤2m/s, it's quite easy for BAB breaking-out.