Quantitative analysis of the error of the chart datum under the variations of mean sea level and harmonic constants can improve the accuracy of sea charts. Year-to-year harmonic analyses were carried out using multiyear monitoring data from the Zhapu, Dinghai, and Jiantiao stations in Zhejiang Province. The mean annual sea level of the three stations shows an increasing tendency with fluctuation, and the maximum interannual errors are 21 cm, 16 cm, and 16 cm. The interannual variations of the amplitudes of the main tidal constituents are not all the same: the relative change of annual constituents is larger than that of semidiurnal constituents, which in turn is larger than that of diurnal constituents. The maximum errors of the amplitudes of the Sa, M2, S2, K1, and O1 components are 13 cm, 20 cm, 10 cm, 3 cm, and 3 cm, respectively. As a result of the instability of the mean sea level and harmonic constants, the maximum errors of the chart datum of the Zhapu, Dinghai, and Jiantiao stations are 33 cm, 26 cm, and 31 cm, respectively, and even if Vladimir J Kis thirteen subtidal algorithm is used, the maximum errors can reach 28 cm, 22 cm, and 28 cm. In contrast, the cumulative frequency of shoals using the theoretical depth datum is below 1%, indicating wide applicability.