Abstract: Currently familiar formulas called normalized correlation, which are used for evaluating the similarity degree of digital watermarks, showed their big shortage when expressing the similarity degree of extracted watermark and original watermark in inverse similarity. Therefore, in this paper, several common formulas of normalized correlation are analyzed firstly. Then by inheriting the rationality and effectivity of these formulas, and overcoming the shortage of them, a novel formula named polarized correlation is constructed, which can more scientifically, rationally, and effectively evaluate the similarity degree of digital watermarks. Lastly, contrasting experiments of polarized correlation and normalized correlation are done, and the experimental results validate the science, rationality, and effectivity of polarized correlation.