Objective When the body posture changed from a standing or sitting posture to a recumbent posture, fluid in the lower body would shift into the head and neck, which could make upper airway narrow and thus cause or aggravate sleep disorders. This study employed a mechanical model and finite element analysis to investigate airway patency. This method could then be used to study the quantitative relationships between the changes in the cross-sectional area of the upper airway, or skin circumference, and the amount of the fluid accumulation in head and neck tissues. Methods We developed two-dimensional mechanical and finite element models with realistic anatomical structures based on magnetic resonance images on an axial plane of the head and neck, which had the narrowest cross-sectional area of the upper airway. We simulated the increase in the amount of fluid in the head and neck by uniformly increasing the cross-sectional area of the soft tissues, and predicted its effect on upper airway size. Results We built finite element models for two healthy males based on their anatomical structures. By comparing model-predicted results with measured data, we validated the soundness of simulating and predicting the effects of fluid shift using such a modeling method. The model-predicted results confirmed the experimental observation that an increase in the amount of fluid in the head and neck could affect the cross-sectional area of the upper airway. The model-predicted results showed that with an increase in the amount of fluid, the cross-sectional area of the upper airway decreased and the skin circumference increased on the selected plane. The models also predicted that the relationship between the changes in the cross-sectional area of the upper airway and the amount of fluid was nonlinear. Conclusions The finite element models could play an important role in studying the effects of fluid shift. The preliminary model-predicted results showed that shifting more fluid into the head and neck during sleep could worsen the narrowing of upper airway and aggravate obstructive sleep apnea syndromes in patients with fluid-retaining states such as heart failure and renal failure.