Over the past few years, numerous photonic-wireless transmission demonstrations with high data rates in the terahertz (THz) band have been reported, aiming to supporting the explosive growth of data traffic in next-generation communication networks. Usually, real-time analysis of THz signals is implemented in the intermediate frequency domain by using electronic down-converters with limited bandwidth, which brings inconvenience to the monitoring of very large THz bandwidth. In this work, an equivalent-time sampling (ETS) system based on a conventional photoconductive antenna (PCA) for capturing and monitoring high-speed THz signals is proposed, theoretically analyzed, and experimentally demonstrated. The PCA, excited by optical pulses from a mode-locked laser (similar to 100 MHz) and biased by the THz fields, enables the equivalent optical sampling of THz signals into low-frequency regions. In our proof-of-concept experiment, a single-frequency signal at 125 GHz and 16-ary quadrature amplitude modulation (16-QAM) signals at a baud rate of 15 Gbaud at 125 GHz are successfully sampled, monitored, and evaluated with a similar to 100 MHz sampling rate. In this way, the high-speed THz signals can be easily accessible with no need of additional high-frequency components. The waveform distortion values for single-frequency and high-speed communication signal are 0.11%, 2.96% (in-phase) and 2.56% (quadrature). Our proposed ETS system is expected to provide a simple, convenient, and straightforward approach to capture and monitor broadband THz signals.