Abstract: Three problems are discussed in this paper.1.The necessary duration of observation for obtaining precise mean sea level values.In order to obtain a mean sea level with an accuracy of ±lcm,in some ports in the Yellow Sea,it would take 16-18 years or nearly 19 years to accumulate accurate mean sea level data with the implication that some tidal stations in the Yellow Sea are located in places where the earthy s crust is in a period of relative stability;while in Bo Hai(Pohai sea)it would take much longer than 19 years,with the implication that the earth's crust is not stable and that in its vicinity there is a possibility of impending large earthquakes or incidence of earthquakes.