A rigorous theoretical derivation on the influence of frequency drift of the carrier has been made in this paper. By analyzing the demodulation formula, the influence formula of frequency drift of the carrier was given. The formula shows that when there is a tiny difference between the frequency of modulation and demodulation signals, the demodulation result will be modulated by a low-frequency signal and the demodulation result will be distorted. Experimental and simulation were carried out on this basis, and the results are completely consistent with the theoretical analysis.