An method for detection of underwater no-reflection object is introduced. It makes full use of the phase-conjugating characteristics of stimulated Brillouin scattering and the optical isolator’s finite aperture, which enhances the signal to noise ratio highly. After passing through the Fabry-Perot etalon, the light reflected by water surface and scattered by water will form different interference rings’ clusters as a result of their different wavelengths. By observing the change of the interference pattern, whether there is a no-reflection object or not in water can be determined. Using photodiode to receive and digital oscillograph to display the signals, the interval between them can be gotten. The depth of the underwater no-reflection object can be calculated. Photodiodes are placed at appropriate position to ensure more higher SNR can be obtained. In experiments, a underwater no-reflection object located at the depth arange of 1-10 m is detected and the results obtained agree well with real condition.