A power supply apparatus with extending a hold up time function includes a transformer, a winding switching unit and a detection unit. The transformer includes a primary side winding and a secondary side winding. A turn number of the secondary side winding and a turn number of the primary side winding have a relationship of a turn ratio. When the detection unit detects that an input voltage of the primary side winding is less than an input predetermined voltage, the winding switching unit is configured to increase the turn ratio, so that an output voltage of the secondary side winding is recovered to an output predetermined voltage.