Unique experimental phenomena are discovered in a large gap semiinsulating(SI)GaAs photoconductive semiconductor switch(PCSS)and the peculiar transmission characteristics are exhibited in the experiment.The transmission characteristics for the large gap SI-GaAs PCSS are entirely different from the commonly designed PCSS.By analyzing the differences of the transmission characteristics between the common and the large gap SI-GaAs PCSS,a detailed statistical analysis and theoretical explanations are expounded.The large gap SI-GaAs PCSS works in the overvoltage relaxation limit space charge accumulation(LSA)mode when the conditions of 5 × 104 s·cm-3 ≤ no/f ≤ 3 × 105 s.cm-3 and noL ≥ 1013 cm-2 must be met in the switch,with no being carrier concentration and f the frequency.The large gap SI-GaAs PCSS we developed has not shown the nonlinear (lock-in)behavior at high bias voltage,so the withstand voltage and service life for PCSS are improved.