The terahertz (THz) radiation properties of low-temperature-grown GaAs photoconductive antenna (LT-GaAs PCA) are studied in order to improve the THz radiation efficiency. The emission spectra of the photoconductive antenna are obtained by THz time domain spectroscopy (TDS) technology. The corresponding frequency domain spectra are obtained by fast Fourier transform (FFT). The results show that the THz signal generated by LT-GaAs PCA is more efficient than that generated by femtosecond electromagnetic pulse from semiconductor surface. The peak of THz electric field increases linearly as bias voltage increasing, and tends to saturation as pump power increasing.