Abstract: A new approach to broadband constant beamwidth beamforming based on Covariance matrix weighting beamforming(CMW)is proposed. Firstly, the working frequency band is divided into a number of narrowband frequency bins,and the covariance weighting matrixes for each frequency bin are deduced using beampattern opthulzation method. With the covariance matrixes, beams of deferent frequencies are same as the reference beam.Subsequently, broadband signals processing is accomplished through syntbe-sizing each frequency bin. This approach solves the problem of distorted signal spectra, improves the performance of the second order statistical methods in broadband system. Computer simulation and lake trials prove the correctness of the proposed method.