A computer for aiding determination of Obstructive Sleep Apnea (OSA) includes a storage device storing with a medical image and a central processing unit (CPU). The CPU executes a method for aiding determination of OSA. The method for aiding determination of OSA includes the following steps. The medical image is obtained. An upper airway model is established. A narrowest cross-section and a nasopharyngeal boundary cross-section are defined in the airway model. A cross-sectional area of the narrowest cross-section and a cross-sectional area of the nasopharyngeal boundary cross-section are calculated. A stenosis rate is calculated according to the cross-sectional area of the narrowest cross-section and the cross-sectional area of the nasopharyngeal boundary cross-section. The stenosis rate is provided. In addition, in the method for aiding determination of OSA, a respiratory flow field simulation may be further performed to obtain and provide a flow field pressure distribution of the upper airway model.