The bunching transport of photoexcited carriers in an GaAs photoconductive semiconductor switch (PCSS) with interdigitated electrodes is investigated under the femtosecond laser excitation. The continuous outputs featured with high-gain (HG) characteristic are obtained at single-shot and 1-kHz by varying the optical excitation energy. An ensemble three-valley Monte Carlo simulation is utilized to investigate the transient characteristics and the dynamic process of photoexcited carriers. It demonstrates that the presence of plasma channel can be attributed to the bunching of high-density electron-hole pairs, which transports in the form of the high-density filamentary current. Results provide the pictures of the evolution of photoexcited carriers during the transient switching. The photoinduced heat effect is analyzed that reveals the relative failure mechanism at optical excitation at repetition rates.