This paper gives a comprehensive analysis and assessment of the impact of climatic conditions on wheat pest outbreak and disease spread, to find out the mechanism how abnormal weather and climate affects the pest and disease occurrence. Reliable data were collected via continuous field monitoring and observation in 20 main wheat-planting provinces, and are related to the climatic traits and meteorologic disasters in the whole development stage of winter wheat during 2009-2010. These year's data are compared with those of past normal years. It is indicated that the unusural meteorologic events, such as the sudden and sharp drop of temperature at the beginning of winter and the long-lasting abnormal low temperature in spring in northern and eastern wheat zones, and historically rare drought through autumn, winter and spring in south-western wheat zones, would postpone the stage of wheat pests and diseases with remarkable diversification of each kind. Wheat stripe rust is restricted to a smaller range and in less serious grade. Wheat sharp spot and wheat powdery mildew occur in a massive area and on a medium grade. Wheat scab and aphids break out seriously at the heading stage; nevertheless wheat mites are lighter than a mid-size one at the former stage. It is concluded that the key stage and the weather elements are different for different pests and diseases, as the bases for classification and focus for forecast. In order to improve the forecast and assessment, it is important to pay attention to weather conditions in both broad and particular areas and for both long and short periods, using the "classification" and "focus" methods.