A power conversion apparatus and an over power protection method thereof are provided. A number of times a detection voltage being greater than a first reference voltage and a number of times the detection voltage being greater than a second reference voltage are counted, so as to obtain a first count value and a second count value, in which the detection voltage is a voltage on a resistor in response to a current flowing through a power switch. Stop switching the power switch when the first count value is greater than or equal to a first threshold value or when the second count value is greater than or equal to a second threshold value.