An initial voltage establishing circuit has a current supplying circuit, a current adjusting circuit, a charge/discharge control circuit, and an oscillating signal generating circuit. The current supplying circuit provides a charge current and a discharge current. The current adjusting circuit adjusts the charge current and the discharge current. The charge/discharge control circuit selectively allows the charge current and the discharge current to flow through. The oscillating signal generating circuit employs the charge current and the discharge current to generate an initial voltage establishing signal such that a duty cycle of the initial voltage establishing signal gradually increases. The initial voltage establishing signal is applied to a switching voltage converter for establishing an initial output voltage.