A protection circuit for power supply is disclosed. The power supply includes a power conversion unit having a primary winding and a secondary winding. The primary winding receives an input voltage. The secondary winding is used for generating an output voltage. The protection circuit comprises a first impedance unit coupled to the secondary winding. A loading current of the power supply flows through the first impedance unit. The output voltage is generated at the first impedance unit. A second impedance unit is coupled to the secondary winding and generates a reference voltage. A comparison unit compares the output voltage and the reference voltage to generate a protection signal for turning off the power supply. A adjustment unit is coupled to the second impedance unit and the comparison unit and adjusts the magnitude of the reference voltage for controlling the comparison unit to output the protection signal.