The numerous Darwin’s harmonic constituents can be combined into four constituents O1, K1, M2, and S2，as Doodson did in [2] and [3]. The amplitudes and speeds of this sort of constituents are no longer unchangeable, and therefore these constituents are called “quasi-harmonic constituent” in the present paper. The major errors of the method suggested by Doodson are (a) to use right ascensions instead of longitudes，(b) to leave the parallax age out of consideration. In this paper, a method with higher accuracy is proposed and shallow-water quasi-harmonic constituents are introduced.