This study researched on the grain yield under the factors of average precipitation, relative humidity, extreme temperatures, and the average temperature of Weifang data. With the methods of principal component analysis, the results showed that the grain yield increased rate slow before 1975 and increased after that, which attributed to the precipitation, extreme temperatures. On the other hand, the frequency of average temperatures and extreme temperature was reduced after 1975 and heat of the sun increased, so the grain yield increased quickly. Except the climate change, the grain yield increased quickly after Reform and Opening Policies. The development of science and technology, new fertilizers, research and development and population flow driven the growth of grain production. In a word, the change of grain yield was influenced with many factors, and the effects of climatic conditions was the fundamental and direct factors.