A power conversion system in an electronic device is utilized for converting an input voltage of a power source terminal to a required voltage of a load circuit to provide power to the load circuit. The power conversion system includes a first voltage conversion circuit for converting the input voltage to the required voltage of the load circuit according to a first control signal; and a power control module for generating the first control signal according to a starting signal or a load voltage of the load circuit; wherein the load circuit receives the voltage outputted from the first voltage conversion circuit to perform operations.