The main tides affecting Dagang sea level are analyzed and the national height datum is studied by analyzing 1980—2011hourly tidal data and 1952—2007monthly mean tidal data.Firstly,the frequencies and amplitudes of main tides including 180short-period tides and 6long-period tides are gained by the Fouirer transform.Then the actual amplitudes and their variations of main tides are obtained by the harmonic analysis of the1980—2011hourly tidal data,and the changes with about 19 year period can easily be found ...