Aiming at the existing problems in the forecast of the prevailing trend of wheat powdery mildew, this paper proposed a forecasting method for wheat powdery mildew prevailing trend based on group analytic hierarchy process (GAHP) to deal with effects of single factor and false on the predicted results. The method introduced multi-source factors which affect the occurrence of powdery mildew, using AHP algorithm to qualitatively analyze the impact factor and prediction scheme for calculating the weight, meanwhile, cluster analysis method was used to calculate the weight of experts. Ultimately, the prevailing trend of powdery mildew was predicted through the weighted sum of the two analysis results obtained. Compared with the predicted results released at Huaibei region Anhui in 2012, this method is feasible and effective.