According to the Mie scattering theory,the backward light scattering power of air bubble appears as clear enhancement near 180° in the condition that the air bubbles are not relevant to each other.The experiment was done with the simulative ship wake. The attenuation characteristic of air bubbles' backward scattered light power was studied with 632.8 nm laser illumination in two conditions-slim beam illumination and expanded beam illumination. The experiment results indicate that considering the attenuation of light intensity as backward scattered light propagating in the water,expanded beam illumination has advantage when air bubbles is nearer and slim beam illumination is adapt to detect farther bubbles.