A new method of array beam broadening is presented in this paper, which uses the differential evolution algorithm combined with Fourier series transform to broaden array beam. Compared with the traditional Fourier series transform, this method can obtain better gain flatness within the range of main beam and lower side lobe level. The presented method can also avoid the problems of having slow convergence speed and easily falling into local optima. The presented method is used to broaden the main beam of a 21 element linear array in this paper. When the elements are excited with the same amplitude and the same phase, the array has a 3-dB beamwidth of 4.02. After using the presented method, the array has a 3-dB beamwidth of 19.6 and the gain flatness within the range of the main beam is 0.21 dB, which validate this method.