IMFs sifted out by EMD are the FM-AM components. In the paper, by the cyclic spectrum analysis, IMFs are demodulated as the two periodic components which are identical to the intrinsic physical concepts. Using the method, the half-day-tide M₂, the day-tide O₁, and the half-month-tide M_f are extracted from the gravity tide signal, and a conclusion is drawn that the half-day-tide M₂ and the day-tide O₁ are primary components of gravity tide signal, and the half-month-tide M_f is modulated in the half-day-tide M₂ and the day-tide O₁.