A power supply system includes a first converting stage, a second converting stage, and a third converting stage. The first converting stage is configured to generate a first voltage according to a first input voltage. The second converting stage is coupled to the first converting stage in series, and includes a first non-regulated power converter and a second non-regulated power converter. The first non-regulated power converter is configured to generate a second voltage according to the first voltage. The second non-regulated power converter is configured to generate a third voltage according to the second voltage. The second voltage is higher than the third voltage, and a varying range of the second voltage is wider than a varying range of the third voltage. The third converting stage is configured to generate a first output voltage according to the third voltage.