A full digital soft-start circuit adapted for a power supply system is provided. The full digital soft-start circuit includes a ring oscillator, a pulse generator, a counter, and a multiplexer. The ring oscillator generates a plurality of clock signals which are different in phase, while equivalent in duty cycle and frequency. The pulse generator generates a plurality of pulse signals with different duty cycles. The counter generates a multi-bit counting signal. The multiplexer determines whether to transmit the pulse signals generated by the pulse generator so as to generate an output pulse which becomes stable as time going on.