The mainly factors of the drought causing disaster are analysed. The results show that the drought disaster occurring mostly depends on the states of the crop water requirement and the environment water supply. The precipitation variability,the water store in soil layers,the crop response on rainfall deficit in the different growing period and hum.an activity influence on the drought disaster. Because the crop sensibility to water in the growing periods are different, the complex evaluation is conducted according to the rainfall deficit, the deficient duration and the crop sensibility to water during the deficient rainfall. The complex index evaluating the drought disaster is also given.