Using theoretical calculations, tidal effects on waves in the Yangtze River estuary were analyzed quantitatively. Considering variations of tidal current and tidal level, wavenumber changing along a wave ray was calculated. In order to avoid errors produced by formula simplification during the calculation, database query method was used. The wavenumbers were used in wave height changing formula, which was deduced based on wave action conservation and in which only Doppler coefficient and shoaling coefficient terms reflecting effects of tide were considered. Then, wave height variation for different water depths and different current speeds of a certain periodic wave along-a-wave ray was obtained, and four examples under different situations were shown. These situations were downstream with low tide, downstream with high tide, upstream with low tide, and upstream with high tide. Their corresponding wavelengths and steepness were also calculated. Through a case study, theoretical results were compared with observations, percentages of tide-induced wave height to total wave height and contribution values of Doppler coefficient and shoaling coefficient to wave height were calculated. The results show that theoretical calculations and observations have good relationship and Doppler coefficient and shoaling coefficient can basically explain wave height’s semidiurnal change. For the study case, tidal effect on wave height is negative, namely, -8% for theoretical calculation and -6% for observation. Effect of shoaling coefficient on wave height is much more important than that of Doppler coefficient. Both theoretical calculation and observation reveal that influence of tidal level on wave height is inferior to that of relative current.