Abstract: Simulated distribution of occurrence and forecast of Therioaphis trifolii at alfalfa planning region in southern Ningxia, using technique of geographical information system(GIS), referring to established forecast model of T. trifolii:y=2 298.017－21.754x₁－47.096x₃＋1.473x₄ (y is the occurrence quantity of T. trifolii, x₁ is June precipitation, x₃ is the density of Coccinella septempunctata in the beginning of June, x₄ is the cardinal number of the T. trifolii in the beginning of June). Technique of area forecast of T. trifolii which is established based on GIS, can precisely reflect pest occurrence range, distribution of region, area and ratio of different occurrence degree. Forecast result of the technique is intuitive and can be extensive application, after sampling investigation, average accuracy of forecast warring is up to 96.6%.