In this letter, we investigate the threshold voltage (V TH ) shift in a p-GaN gate AIGaN/GaN transistor by designed gate-bias pulse measurements. It was found that the forward gate bias causes positive (V TH ) shift. The dynamics of electron trapping was revealed from the dependences of the consequent (V TH ) shift on the bias duration at different voltages. A time constant smaller than 0.1 ms for the (V TH ) shift saturation at 6-V gate bias was obtained. It was also found that the (V TH ) became inversely proportional to the gate-bias voltages exceeding 7 V. This inverse proportionality of the (V TH ) shift resulted from the threshold of the hole-injection/electroluminescence (EL) and the sequential optical pumping effect on the electron traps. The EL emission was confirmed by a self- and in-situ photon detection measurement.