In order to quantitatively evaluate the crop disaster losses caused by drought in northwest of China in the context of climate change, a Bio-meteorological Index-DI, that is used to quantitatively evaluate the drought loss of spring corn in Gansu Province, China, is built. The DI index consists of CI index and water fitness. The former reflects the role of the medium- and long-term precipitation and heat condition; the latter represents the relationship between the water supply and physiological water demand of corp. According to the drought-reduced and growing season precipitations, the DI index is divided into four degrees, that is, mild drought, moderate drought, serious drought, and particular heavy drought, respectively based on the verification analysis and feasibility evaluation of the DI index. The historical daily DI index and accumulated DI index are backward calculated by using the materials of daily precipitation and temperatures collected from the meteorological stations, the corn yields and total productions, developmental and physiological water demand. The temporal and spatial variations of spring corn DI index for the representative stations in the growing areas have some practical significance on the agricultural crop system adjustment, crop allocation, and disaster prevention and mitigation.