Based on Maxwell equations，the theoretical investigation on terahertz far-field radiation from large-aperture photoconductive antennas triggered by femtosecond optical pulses is presented. The frequency spectrum characteristics of THz pulse from large-aper ture GaAs photoconductive antennas with different electrode shapes and gaps are analyzed. Different GaAs photoconductive antennas are fabricated and experiments are performed. The experiment results and theoretical analysis show that the bandwidth of large-aperture photoconductive antennas is 0.1 THz-2 THz；the different electrode shapes mainly effect the THz radiation amplitude in 1 THz-2 THz；smaller gap results in wider frequency and higher amplitude above 1 THz.