This paper presents our investigation into a 220-GHz signal monitoring system.The detectable frequency band of the proposed system is 220 to 230 GHz with the ability to distinguish mirror frequency. The detectable signal ranging from one tone signal to 1 GHz and the frequency accuracy is 1 KHz. More importantly, the terahertz monitoring system can accurately estimate the center frequencies of monotone signals and common modulation signals. The modulation methods including BPSK, QPSK, 8PSK, 16QAM, 32QAM, and 64QAM.The front end of the receiver consists of a 220-GHz subharmonic mixer and a 110-GHz tripler that are based on Schottky diodes. Both the mixer and the tripler show great performance and guarantees the well operation of the monitoring system. In addition, the baseband algorithm is modified to improve efficiency and reduce computation. The experiment of signal detection shows the system can identify the frequency, power, bandwidth, and the modulation mode of the unknown signal during the operation band.