A self-excited power conversion circuit for secondary side control output power includes a comparator unit and a transistor installed directly in a secondary side output module, and the comparator unit is electrically coupled to at least one load, and the transistor is electrically coupled between to a conversion module of the circuit and the load. The comparator unit is provided for adjusting the duty cycle of the transistor after detecting the amount of energy outputted from the conversion module to the load from, so as to adjust the amount of energy actually received by the load to achieve a constant power effect.