A constant water level humidifier has an outer tank with an outer outlet disposed at the bottom, the outer outlet is disposed with an oscillating plate, the upper portion of the outer tank is disposed with an outer air vent connected to the outer world; wherein the outer tank is further disposed with an inner tank, the upper portion of the inner tank is disposed with an inner air vent connected to the outer air vent, the bottom portion of the inner tank is disposed with an inner outlet connected to the outer outlet of the outer tank and an inner inlet connected to the chamber of the outer tank, the inner inlet is disposed with a water level control mechanism.