AC gas‐liquid discharge is attractive for on‐site metal detection. Intensified charge‐coupled device‐based temporal‐resolved diagnosis improves detection performance for a pulsed system but fails for AC discharge due to the temporal randomness and absent trigger signal of discharge. Herein, a trigger‐free response system based on a high‐speed photomultiplier tube is proposed to perform temporal‐resolved diagnosis for AC gas‐liquid discharge. It improves the detection limits for Cu, Mg, Na, and K by more than an order of magnitude. This work provides a perspective on diagnostic techniques that enhance trace metal detection performance based on AC gas‐liquid discharge.