Disclosed herein are a group of gastric cancer VOC markers in saliva and an application thereof in the preparation of a diagnostic reagent of gastric cancer. The markers are a combination of compounds selected from the group consisting of acetaldehyde, 2-methylbutyraldehyde, isopropanol, hexanal, n-butanol, cineole, nonanal, menthone, 2-ethylhexanol, menthol, anethole and dodecanol. The diagnostic reagent is used for detecting the contents of the marker in a saliva sample of a subject to perform the diagnosis of gastric cancer.