The invention discloses a gas chromatography method for detecting C18 fatty acid isomers in grease. A high-polarity ionic liquid gas chromatographic column with the specifications of 200 m*0.25 mm*0.20 mu m is used for detecting the C18 fatty acid isomers in the grease. By means of the method, the separation degree of C18:1 and C18:2 isomers is improved and C18:3 isomers are effectively separated. According to the established C18 fatty acid isomer method, the RSD (relative standard deviation) of the within-day precision is controlled in the range of 0.83%-4.30%, the RSD of the day to day precision is controlled in the range of 1.53%-3.90%, and the quantitative analysis requirements are met. According to the established C18 fatty acid isomer method, the detection limit ranges from 0.009 mg/L to 0.075 mg/L, and the minimum determination limit ranges from 0.028 mg/L to 0.225 mg/L, and the detection limit and the determination limit of C18 cis-trans-isomers are remarkably reduced.