Abstract: A channel quality estimating method is proposed, which can be used to quantitatively analyze the influence of time-invariant channels with different multipath structures on the single-carrier underwater acoustic (UWA) communication performance. Since the correlation between the residual mean square error of the signal linear equalization under UWA channels and the bit error rates (BERs) of the communication system is strong, the residual mean square error boundary of the signal linear equalization is calculated and defined as a variable only related to the channel impulse response, namely Wiener error boundary (WEB), then it is utilized as a quantitative indicator of the qualities of multipath channels to evaluate the impact of multipath channels on the communication performance. To verify the performance of the method, the correlations between various quantitative indicators of sea trial UWA channel qualities and BERs after channel equalization are analyzed to compare the performance of corresponding indicators. The experimental results indicate that, compared with traditional indicators such as delay spread, coherence bandwidth, spectral null depth and signal multipath ratio (SMR), the Spearman correlation coefficient between WEB and BERs is higher, and the average correlation coefficient can reach 0.71. WEB can be used to quantify and classify multipath channels of different quality effectively, and reflect the impact of UWA channels with different multipath structures on communication performance quantitatively.