A gas chromatography-internal standard method was developed to measure the contents of fatty acids in four types of pre-prepared foods. The food samples were hydrolyzed with hydrochloric acid, extracted with ether-petroleum ether, saponified with 2% sodium hydroxide methanolic solution, and methylated with 15% boron trifluoride methanolic solution. Then, they were diluted with isooctane and layered with saturated sodium chloride aqueous solution. Finally, water was removed with anhydrous sodium sulfate, and the upper layer was filtered through a 0.45 μm membrane. Separation was performed using a DB FastFAME (90 m×0.25 mm, 0.25 μm) chromatographic column with a gradient temperature program, and an internal standard method was used for quantification. Under the established conditions, 36 fatty acids were effectively separated. Good linearities were observed in a certain range, and the correlation coefficients were higher than 0.99. The detection limits and quantification limits were 3~6 mg/100 g and 10~20 mg/100 g, respectively. Validation by spiking showed that the average recoveries at three levels ranged from 72.43% to 94.54% and relative standard deviations ranged from 2.84% to 8.71%. The proposed method demonstrated high accuracy, good reproducibility, and a low cost for detection, thus proving effective for measuring fatty acid content in pre-prepared food. Fatty acid content was measured in four pre-prepared foods: Yu Xiang shredded pork, braised lion’s head meatballs, black pepper beef strips, and braised chicken with yellow sauce. This method can be used to accurately determine fatty acid content in various pre-prepared foods. It provides technical support for the market supervision of pre-prepared dishes.