Using the observational data of Wusong, Gaoqiao, Puzhen, and Luchaogang tidal stations of the Yangtze River estuary in recent years, and by the method of single factor linear regression, the relationships between the mean tidal level and the mean half-tide level were analyzed. The results showed that all stations have very good linear relationship between these levels. Based on this relationship, the mean tidal level can be calculated by using the high and low tide data. As the sea level of certain sea area is calculated using its representative station average of the whole point tide level in certain time, in case of the lack of long series of tidal data, the method here can help us to calculate the mean tidal level indirectly, and further to calculate the sea levels effectively.