A level adjuster of a water supply of a water box serves for adjusting the amount of water in a water box conveniently and easily. The level adjuster comprises a press, a supporting rod, an pressable button, a positioning seat, a pontoon, an inner water injecting tube and an outer water injecting tube of a water injector. The amount of water in the water box can be precisely controlled and the operation is easy without using any other auxiliary tool. No problem of element fatigue and change of water level occurs.