The invention discloses a method for determining aqueous tobacco extract by adopting an online infrared-assisted hydration reaction head space-gas chromatography/mass spectrometry. The method comprises the following steps: (1) placing a test tube with aqueous tobacco extract into a head-space sample bottle, sealing a bottle opening, and placing a drying agent at the inner bottom of the head-space sample bottle; and (2) heating the head-space sample bottle by using an infrared lamp until vapor in the aqueous tobacco extract is completely absorbed by the drying agent, then carrying out the head space-gas chromatographic/mass spectrometric determination, calculating according to a peak area normalization method, and determining a relative percentage composition of each component. The detection method is simple, accurate and sensitive in operation, has a popularization and application value, can be effectively used for analyzing aromatic sample containing moisture such as essence and perfume, and is particularly suitable for head-space feeding solid samples containing moisture and liquid samples or solid samples containing a component capable of reacting with the drying agent.