A method of automatically adjusting a determination voltage used in an induction type power supply system includes detecting an output voltage of a signal analysis circuit; adding a first threshold value to the output voltage to generate a first determination voltage and subtracting a second threshold value from the output voltage to generate a second determination voltage; outputting the first determination voltage as a reference voltage; and comparing a trigger signal of the signal analysis circuit and the reference voltage, in order to generate a first data code; wherein when the step of comparing the trigger signal of the signal analysis circuit and the reference voltage in order to generate the first data code fails, the method further includes outputting the second determination voltage as the reference voltage and comparing the trigger signal of the signal analysis circuit and the reference voltage, in order to generate a second data code.