Abstract: It is difficult for passive sonar using traditional broadband energy detection method to detect the weak target in the presence of strong interferences. Based on different spectrum characteristics of target, interference and background noise, a new method by dividing sub-band of spectrum and using color synthesis theory is proposed to achieve broadband fusion detection. When the radiated noise of weak target contains stable line spectrums or its spectrum energy at some frequency points is relatively strong, the ability to detect weak target can be effectively improved by this method, and the bearing-time-records of different targets can be easier distinguished.