Abstract: This article analyzes and studies the constituents of fatty acids in marine fish oil by gas chromatography.It introduces ways such as to discern the false fish oil from the genuine by the characteristic indexes of fatty acids in fish oil;to distinguish the cheap animal and plant oils from fish oil by the difference of the constituents of fatty acids;to determine the false degree by testing the deviation of the amounts of fatty acids such as C18:2,in constituents which in false oil is different from the characteristic indexes in fish oil.It effectively discerns the false oil from the genuine in the condition that the kinds and sources of samples is unknown.The method is simply and convenient.It can be used to appraise the quality of feeding fish oil.