Objectives: The objective of this study was to explore the relationship between seat armrest height and human neck comfort when using a smartphone while sitting on a passenger seat during the flight. Material and Methods: The authors used a wireless angle-measuring instrument and subjective comfort scale to evaluate the changes of head flexion angle and neck comfort level of 24 young smartphone users in an aircraft simulated cabin. Results: The study results indicated that using a smartphone while sitting on a passenger seat during the flight would pose a larger discomfort to the neck, and the discomfort would be higher for gaming than reading tasks. Seat armrest height is related to the comfort level of the neck when using a smartphone, increasing the height of the armrest can effectively alleviate discomfort in this state. Conclusions: Considering the prevalence of passengers using smartphones in aircraft, a seat armrest that can be properly adjusted in height, which can effectively reduce the risk of passenger head flexion angle and neck discomfort. Int J Occup Med Environ Health. 2022;35(2):199–208