The accurate detection of harmonic content in the power network of the ships electric propulsion system is very important. In order to further improve the accuracy of harmonic measurement, this paper improve the Blackman window function, and use the bimodal spectral lines interpolation algorithm to detect the power signal of marine electric power system. Use the software to approximate the parameters of improved window function, and set up the experimental platform. By comparing the measured results of adding different Window functions, we can get the conclusion that the improved Blackman window can effectively improve the accuracy of the harmonic detection.