Provided is a humidifier (14) used to humidify breathable airflow which is to be transmitted to a patient, comprising a chamber (16) configured to receive breathable airflow, a tank (20) configured to hold supply water and arranged to be inserted into the chamber, a cover (18) arranged on the chamber and able to move between an open position and a close position, a sealing element (19) arranged on the cover. The sealing element is configured in a sealed cavity, so that the breathable airflow pressurizes the cavity. The humidifier can be connected with an airflow generator (12) which is configured to generate breathable airflow. A tube used to deliver humidified airflow can be connected with the humidifier. The tube can be a heating tube or a non-heating tube. A breathing device used to deliver breathable airflow to a patient comprises a humidifier and an airflow generator and/or a tube. The method for humidifying the breathable airflow comprises sealing the cavity which holds the supply water, and pressurizing the cavity through guiding the breathable airflow into the cavity.