Beampattern synthesis plays a critical role in fixed beamforming. A fast beampattern synthesis method is highly desired, especially for broadband beamformers requiring a large number of weight parameters for better performance. This paper proposes a low-complexity broadband beampattern synthesis method for time-domain beamformers. Introducing the null-forming scheme of adaptive beamformers, a virtual interference-plus-noise matrix is iteratively constructed to control the sidelobe pattern accurately. The proposed method reduces the computational complexity when compared with the existing algorithms based on the interior-point method (IPM), especially for large-scale microphone arrays. Simulation results demonstrate that the proposed method obtains equivalent beampatterns with much higher efficiency than the IPM-based method. In speech extraction experiments, the designed beamformers exhibit better suppression performance than the conventional fixed beamformer.