An introduction is made to the design, development, and core technology of the weatherbased graded wheat scab forecasting system based on Geographic Information System, taking Jiangsu Province as an example. The system uses VB.Net 2005 and ArcEngine as a development platform, based on MSSQL SERVER 2005 database query, using such three meteorological factors as daily average relative humidity, daily mean temperature, and the daily sunshine hours. Two algorithms are used to calculate meteorological grades for wheat scab occurrence and development, and text and graphics products containing geographic information are generated, so to achieve the capability of forecasting wheat scab. The system has the characteristics of powerful capability, efficient algorithm, and simple operation, with good potentials in application.