The seasonal variability of M₂ constituent exerts a major influence on the coastal ocean environment. The enhanced harmonic analysis (EHA) can synchronously extract the temporally varying amplitudes and phase lags of significant constituent and the constant values of the other constituents. In this study, the seasonal variability of the M₂ constituent in the Bohai Bay, China, is investigated by analyzing one-year sea level observations at two stations with EHA. In order to evaluate the accuracy of EHA, artificial "sea level" is designed in the ideal experiment. Both the estimated temporally varying amplitude and phase lag of the M₂ constituent and constant values of the S₂, K₁ and O₁ constituents using EHA were much closer to the prescribed values than those obtained using the other methods, indicating the capability and efficacy of EHA. When the real sea level observations were analyzed using EHA, the estimated M₂ constituent amplitude has significant seasonal variability, with large values in summer and small values in winter. The sensitivity experiments show that the trend of seasonal variation of M₂ amplitude in Bohai Bay is not affected by the experimental settings, and it is robust and can reflect the real seasonal variation of M₂ constituent. The seasonal variations in the M₂ amplitude is possibly induced by the seasonally alternating Asia monsoon, as the Asia monsoon dominates the seasonality of mean sea level, stratification and eddy viscosity, which are main influence factors summarized in the previous studies.