A method of using frequency selective surface (FSS) to dynamically control beamwidth of antenna is proposed. The unit cell is composed of a two-layer periodic H-shape structure loaded with varactors. By tuning the bias voltage applied to the varactors, the unit cell has two different states that are transparent and absorptive to the incident wave at a certain frequency, respectively. We can control the transparent region of the whole FSS through reasonably setting the capacitance of the varactor loaded on each unit cell, and thus the beamwidth of the transmitted wave could be tuned. The above property has been validated by putting the FSS above the horn antenna array as superstrate. Simulation results show that E-plane beamwidth of the antenna can be enlarged from 13.2 degrees to 31.1 degrees at 5.5 GHz, and experimental results agree well with the simulated ones. In addition, it is found that the operation frequency for beamwidth control can be tuned as well.