Abstract: Objective To establish a regional seamless chart datum and achieve seamless vertical datum conversion,thispaper starting from the datum calculation principle,and according to the tidal wave propagation characteristic,we propose a new method of constructing regional seamless chart datum based on the tidal harmonic constantsinterpolation.Firstly,the constants of unknown points in waters can be calculated by interpolation with usingthe known tide gauges.Secondly,we establish seamless chart datum based on these points with using a math-ematical model.When compared with the traditional geometric method,it significantly improves the construc-tion precision and overcomes the precision unstablity defect.Experiments in Yangtze Estuary were performed,and the results show that the method is correct and feasible.Finally,a seamless chart datum in Yangtze Estu-ary was established.