A gas exhaust control method applied to a respiratory assistance apparatus includes the following steps of: applying gas to a face mask of the respiratory assistance apparatus; detecting a pressure in the face mask; when the pressure in the face mask is lower than a preset pressure range, shrinking a perforation size of an exhaust structure of the face mask by a driving unit of the respiratory assistance apparatus; and when the pressure in the face mask is higher than the preset pressure range, expanding the perforation size of the exhaust structure of the face mask by the driving unit. A respiratory assistance apparatus applied with the gas exhaust control method is also disclosed.