A novel power-rail electrostatic discharge (ESD) clamp circuit was proposed, which aimed to solve the issue of the insufficient turn-on duration of RC trigger-mechanism under the small capacitance condition. This circuit used a method of feedback and shunt mechanisms combined with a capacitance, in which a small capacitance was in parallel to a shunt nMOS controlled by another feedback nMOS. The shunt nMOS diverted the capacitance charging current, so the turn-on duration of the circuit was prolonged. The simulation results show that the turn-on duration of the proposed circuit is enough long, and can respond quickly to the ESD event, which can achieve ESD protection. Furthermore, the proposed circuit with an adjustable minimum starting voltage (Vstarting) could effectively avoid the false trigger under the fast power-on condition.