In order to research the benthic algae within the coverage of offshore 50m wide of Gouqi Island, on the one hand we acquired echo samples around Gouqi Island by BioSoics DTX. On the other hand, we predicted the benthic algae coverage by inverse distance weight (IDW) interpolation for nonsampled area. This method which combines echo sounding and interpolation is economic and highefficient. The results of the study show that benthic algae around Gouqi Island are distributed patchily. Interpolation makes up for the shortcoming that echo sampling cannot cover to reveal the total resources in the entire research range. While, “Closer in Space，more Similar in Coverage Degree” is the specific character of IDW interpolation, this fits well with the patchily growing benthic algae to predict benthic algae resource. When given a perfect power exponent, the IDW interpolation can predict and assess the distribution of benthic algae effectively, in this research the most suitable power exponent is p=5. Now, the applications of Echo Sounding and Interpolation in assessing benthic algae resource are at the very beginning, even more, most of them are conducted in calm lakes and rivers and only a few similar studies have been done in the ocean environment. At the present stage, deeper research is much needed. This paper digs and develops the application characteristics while conducting Echo Sounding and Interpolation Method to assess the benthic algae resources in complicated ocean environment.