Abstract: In order to timely prevent and effectively control the occurrence of pear scab, a portable infection forecasting and alarming device, which could real-time forecast the Venturia nashicola infection quantity, was designed. By making use of the core component MCU STC89C52RC and the temperature and humidity intelligent sensor SHT15, the real-time data acquisition of temperature and humidity was realized. The relative infection quantity of Venturia nashicola was real-time calculated according to the acquired temperature and humidity, and the real-time forecasting and alarming of Venturia nashicola was realized by using of the infection quantity. The forecasting and alarming device has the feature of simple operation, friendly interface, convenient carry and high performance-cost ratio. Experiments have demonstrated that the designed forecasting and alarming device has a high forecast precision, which provided a scientific basis for preventing plant diseases timely for fruit growers.