An over charge protection method applied to a voltage converter which can operate in a quaternary modulation mode (Q mode) or a ternary modulation mode (T mode). The over charge protection method comprises: (a) determining whether the voltage converter operates in the Q mode or the T mode; and (b) setting a current threshold of the voltage converter to a first over current threshold if the voltage converter operates in the T mode; and (c) setting the current threshold to a second over current threshold if the voltage converter operates in the Q mode, wherein the first current threshold is smaller than the second over current threshold.