The present invention discloses a negative-pressure oral apparatus capable of relieving discomfort of soft tissues in the oral cavity and improving fixation of the oral apparatus to prevent the oral apparatus from falling off due to mouth opening. The oral apparatus is compact and elastic so that it fits various shapes and sizes of the oral cavity and is easy to be put on and taken off to provide convenience and safety to the user. In another embodiment, a method for maintaining a negative oral pressure and collecting liquid by coupling the oral apparatus to a liquid collecting apparatus capable of effectively providing the oral cavity with a negative pressure to expel liquid in the oral apparatus and deliver the liquid to an absorbing element in the liquid collecting apparatus so as to prevent liquid leakage or contamination.