A method of switching operational modes of a power converter includes starting up a primary controller applied to the power converter to make the power converter operate in a normal mode; a secondary controller applied to the power converter detecting an output voltage of a secondary side of the power converter and the primary controller detecting an operational frequency of the power converter; if the operational frequency of the power converter is lower than a predetermined frequency and satisfies a first frequency condition, the primary controller controlling the power converter to enter a sleep mode; and when the power converter enters the sleep mode and the output voltage is lower than a trigger voltage, the secondary controller making the primary controller to control the power converter turned on.