Abstract: The gain of the open-ended rectangular waveguide was studied and the effects of flange on the gain of waveguide were discussed. The gain of BJ100 was calculated by an electromagnetic field simulation code. It is found that the period of the gain fluctuating with waveguide length is half wavelength. Then in the simulation, the outer wall of the waveguide was pasted with absorber to eliminate the effects of flange on the gain. The numerical simulation shows that this method weakens or even eliminates the dependence of gain on frequency, and the major lobe of the directional pattern becomes much wider with a peak in the center.