The technique of terahertz pulses generated from the photoconductive switches has been applied in the ultrafast electrical pulse metrology recently.A lumped-element theoretical model is established to describe the performance of the LT-GaAs ultrafast photoconductive switch used in the ultrafast pulse standard.The carrier transport processes of the photoexcited semiconductor,the attenuation and dispersion during terahertz pulse propagating are considered in the theoretical model.According to the experimental parameters,the waveforms of the generated terahertz pulses are calculated under optical excitations with different wavelengths of 840 nm and 450 nm,respectively.And comparisons between the theoretical results and the experimental results are carried out. The technique of terahertz pulses generated from the photoconductive switches has been applied in the ultrafast electrical pulse metrology recently. A lumped-element theoretical model is established to describe the performance of the LT-GaAs ultrafast photoconductive switch used in the ultrafast pulse standard. The carrier transport processes of the photoexcited semiconductor, the attenuation and dispersion during terahertz pulse propagation are considered in the theoretical model. According to the experimental parameters, the waveforms of the generated terahertz pulses are calculated under optical excitations with different wavelengths of 840 nm and 450 nm, respectively. And comparisons between the theoretical results and the experimental results are carried out.