In this study, a rapid and sensitive method to determine the flavor precursors was established on the basis of modified Kubec's gas chromatography (GC) method. For the new method, the interference of halogen was removed by adding borohydride that replaced halide as a reducing reagent, and the analysis was completed within 25 min for the determination of S-alk(en)yl-L-cysteine sulfoxides (ACSOs). In comparison with the results of high-performance liquid chromatography (HPLC), the modified GC method is much better in the separation and detection of ACSOs in yellow onion bulbs than the original method. A small amount of 2-propenyl-L-cysteine sulfoxides (2-PeCSO) was detected with the modified GC method, and propyl-L-cysteine sulfoxides (PrCSO) were not detected. The results of GC determination indicated that 2-PeCSO probably was the hydrolysate of a byproduct or an isomer of gamma-glumtyl-1-propenyl-L-cysteine sulfoxides in biosynthesis process.