Abstract: In order to accurately measure the water activities (a_w) of heated tobacco products, a method using a water activity meter with an electrolytic sensor was developed. The variations of water activity and microbial counts in heated tobacco products as a function of storage time were studied. The results showed that: 1) The electrolytic sensor could eliminate the interference caused by the volatilization of propylene glycol in the tobacco. The optimal test conditions were 25 ℃ sample temperature and greater than 0.5 g sample mass. Dispensing with shearing and grinding, sample preparation could be carried out quickly in the relative humidity environment close to the water activity of the sample. Test samples should be sealed and detected immediately after unsealing. 2) The intra-day and inter-day precisions of the method were both less than 1%. The water activities of 35 heated tobacco product samples ranged from 0.43 to 0.55 with an average of 0.46. 3) With prolonged storage, the water activities and CFU did not change significantly in the tobacco materials with different initial water activities, while the mold counts decreased. The tobacco materials of heated tobacco products with an initial water activity of 0.806 displayed white spots after storing for 15 d, which was not caused by mildew and might be formed by the physical change due to excessive moisture content.