Abstract: Four computation methods of first order differential quotient value for the theoretical earth tide-wave separation, first order difference, zenith distane differential and interpolation differential-have been introduced. The results of various methods compared with differential quotient values and fitting testhave been shown. The interval of first order differential method and the interpolation differential method are discussed. The results have shown that the optimum computation method of first order differential quotient values for the theoretical earth tide is the zenith distance differential method or the interpolation differential method in the fitting test of the earth tide data, and the value of △T should be greater than 1 sec in difference method.