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Objective To propose a magnetic resonance imaging（MRI）procedure for 3D volume scanning to study the effect of fluid shift on the cross section of the upper airway as well as the neck circumference，and to test the feasibility of this strategy. A new thought was provided for exploring the effects of body posture changes on the respiratory functions during sleep. Methods A procedure using 3D-SPGR scanning from the top of nasal cavity to laryngeal prominence of two subjects was conducted under two conditions：the control test with subjects lying supine，and after fluid redistribution resulting from elevating both legs for 8 min. The coordinates of the neck and the upper airway boundaries were extracted directly from the selected axial images to calculate the neck circumference and the cross-sectional area of the upper airway. Results The images showed clearly the changes in the neck circumference and the cross-sectional area and shape of the upper airway. The results indicated that the fluid shift caused by raising legs could narrow the upper airway and increase the neck size，which were consistent with currently published experimental results by other investigators. Conclusions Using the 3D volume MRI technique to study the effects of fluid shift on the upper airway patency，we can easily and accurately measure the changes in the neck size and the cross-sectional area and shape of the upper airway. This method could provide an effective way to investigate the effects of body posture changes on the respiratory functions during sleep，as well as the mechanism behind the correlation between fluid-retaining related diseases and obstructive sleep apnea.