The causation of MFJE (Modulation Frequency Jitter and Excursion) is generally introduced,and the error of MFJE in coherent detection is analyzed at the same time, the mathematic models of MFJE error in analog and digital coherent detection process are built. Based on the mathematic models, the effects of MFJE on analog and digital coherent modulation-demodulation process are identified, especially the effect of MFJE on discrete sampling. Finally, a Membership Weighted Function Sampling (MWFS) algorithm is proposed. The experimental result shows that the precision of coherent detection has not been affected in a modulation frequency excursion range of ± 9.2 % with MWFS,compared with the range of ±3% without MWFS. The result also shows that the mathematic models of MFJE error in analog and digital coherent detection process are valuable in coherent detection application field and MWFS proposed in the paper can eliminate the effect of modulation frequency excursion on coherent detection efficiently, which has been certified in practice.