In order to analyze the cause of abnormal peaks in Microwave Bragg Diffraction Experiment withlarge incident angle,theoretical analysis is carried out,the threshold incident angle and theoretical angles otabnormal peaks are calculated.When solving the threshold incident angle,the existing method is improved,theinfluence of the horn angle of the receiving horn is considered, which makes the calculated value of thethreshold incident angle more accurate , successfully explains all the abnormal peaks in the experiment andverifies the analysis of the cause of the abnormal peaks.