As for semi-insulating GaAs photoconductive semiconductor switch (PCSS) operated in lock-on mode, threshold conditions of biased electric field and optical excitation energy are measured by a pulsed circuit. Experimental results show that the field threshold has an analogy to inverse exponential dependence on the optical energy threshold. PCSS can be triggered into lock-on mode with 9 kV/cm when the optical energy is on the order of 10 μJ; and with the optical energy above 0.78 mJ, PCSS will be operated in linear mode but not lock-on mode for different voltages. On basis of the results, high-gain dipole domain model is proposed to analyze electric field and optical energy thresholds of the PCSS. The results from the experiments and calculation are in good agreement within the measurement error.