The invention mainly provides a power supply regulator, comprising a P type power transistor, a feedback circuit, a differential amplifier, a protection circuit containing an N type transistor current mirror and an active foldback current limiting circuit without containing a resistor. When the P type power transistor undergoes short-circuit current, the active foldback current limiting circuit increases the current at the output end of the direct current mirror in the protection circuit to restrict the current flowing through the P type power transistor. The invention also discloses the active foldback current limiting circuit which can increase the current at the input end of the direct current source in the protection circuit to achieve the same aim.