Abstract: In order to accurately determine the release of moisture and tar in the aerosol of heated tobacco products, an in situ extraction device with a smoking machine trap was designed and tested. The aerosol produced by machine-smoking a heated tobacco product was collected, the Cambridge filter pad inside a bespoke metallic trap was directly extracted and then the moisture was analyzed by GC. The results showed that: 1) The recoveries of the in situ extraction method were in the range of 95.4%-104.6% and the relative standard deviations (RSDs) ranged from 3.72% to 6.09% with the limit of quantification of 0.16 mg/cigarette. 2) Comparing with conventional vibrating extraction, the moisture content in the aerosol determined by the in situ extraction method increased by 15.95%-22.16% under the ISO puffing regime and 38.34%-40.37% under the HCI puffing regime. 3) The main components, such as nicotine, 1, 2-propylene glycol and glycerol, in the aerosol of the tested heated tobacco products were not affected by the in situ extraction method. This method was also applicable to the determination of tar release in the smoke of conventional cigarettes. This method could be used for the accurate determination of moisture, tar, nicotine, 1, 2-propylene glycol and propanetriol in the aerosol of heated tobacco products.