A new design method of wideband beam-former with constant beam-width is proposed: within every frequency bin, assume all weighting sequences of beams oriented to different directions are the same, and then it can be deduced that the sequences in all sub-bands have the same spectrum, but a scale relationship is satisfied on the frequency axes. The scale is equal to the ratio of the central frequency in each sub-band to the reference frequency. Based on this idea, a method to calculate the beam-forming matrix is presented: supposing the array aperture changes with frequency, the spectrums of weighting sequences in all sub-bands can be calculated by Chirped Z Transform (CZT), and then they can be obtained by inverse fast Fourier transform (IFFT). The validity of these conclusions is validated with computer simulations.