An ultra-low-power mode control circuit for a power converter includes four modules: a level shift circuit, a start circuit, a static clamp circuit, and a control circuit. When a chip is powered on and a core voltage has not been established, the control circuit firstly starts a power source built-in clock to support operation of the power converter. When the core voltage is established, the control circuit determines whether to switch to an external clock according to a level of a mode selection signal. After the core voltage is powered down, the control circuit automatically wakes up the built-in clock to work.