The present invention provides a method and apparatus for treating obstructive sleep apnea by using negative oral pressure to a patient. The present apparatus includes a vacuum unit for controlling and maintaining negative pressure of an oral cavity of the patient, a tube with one end thereof connecting to the vacuum unit to suck out air in the oral cavity to generate the negative pressure therein, and a mouthpiece connecting to the other end of the tube and fitting into and sealing the patient's mouth to prevent the oral cavity from air leakage. By using negative pressure in the oral cavity, the patient's soft palate is pulled toward the oral cavity and the patient's tongue is pulled toward an upper palate so as to maintain the patient's nasal air passageway open.