Abstract: A radiated noise measurement method of underwater acoustic targets with relatively low power by a vertical nested array is studied. Firstly, the array gain requirements for different frequency bands are obtained through comparative analysis of the background noise level and the radiated noise level of the target. Secondly, an ultra wideband constant-beamwidth beamforming method with high robustness is established for the waveguide environment based on convex optimization. The radiated noise measurement and the following spatial array signal processing procedure is formulated. Finally, a vertical nested array measurement system is developed, and the radiated noise measurement method is verified using lake test data. Experimental results show that the radiated noise measurement method has large working bandwidth, high robustness and low measurement uncertainty of about 2 dB.