Abstract: To effectively monitor the water activity (a_w) of heated tobacco products, a method for determining a_w of heated tobacco products was developed using a tunable laser sensor. Detailed methodological studies were conducted based on sample preparations and storage conditions, test conditions, and collaborative experiments. The results showed that: 1) The method could avoid the interference of atomization agents and menthol, and had good selectivity. 2) Sample materials did not need to be homogenized, and the environmental exposure time should be controlled within 1 min. After unpacking, the samples could be stored for 3 h at room temperature or 30 d at －18 ℃ under sealed conditions and needed to be returned to room temperature prior to testing. 3) The optimized measurement conditions were as follows: the preferred sample mass 0.8－1.0 g for shredded sample and 2.0－4.0 g for granular sample, using an a_w variation value of less than 0.001 within 2 min as the reading endpoint, and testing twice. 4) When applied to different heated tobacco product materials, the measurement results were satisfactory with regard to repeatability limits (r) ranging from 0.002 7 to 0.006 1 and reproducibility limits (R) ranging from 0.004 5 to 0.015 5 on seven representative heated tobacco product samples in a five-laboratory collaborative experiment.