Abstract: In the microwave Bragg diffraction experiment,the reason why singular peak is caused by the opening angle of emission horn on simulation crystal（100） surface has been analyzed.Because microwave emitted by horn is not strictly plane wave,there exists a critical grazing angle when Bragg diffraction happens.When the grazing angle is less than the critical angle,receiving horn can not only received the crystal surface reflection wave,can also receive directly microwave emitted by horn,their coherence stack arises singular peak.The grazing angle corresponding to singular peak has been calculated,and it is consistent with the experimental data.

Key words: microwave, Bragg diffraction, singular peak