An I/O pad ESD protection circuit is composed of a SCR circuit, a first diode, a second diode, and an anti-latch-up circuit. The SCR circuit has a first connection terminal and a second connection terminal, respectively coupled to the I/O pad and the ground voltage, so as to discharge the electrostatic charges. The anti-latch-up circuit has two terminals, which are respectively coupled to the voltage source and the ground voltage, and another connection terminal, used to send an anti-latch-up signal to the SCR for changing the activating rate. The latch-up phenomenon is avoided.