Terahertz direct modulation technology is different from the traditional mixing architecture of the communication system, in order to achieve higher speed rate while having a very simple system architecture. As an all-digital terahertz communication technology, the direct modulation terahertz communication system has a high commercial application prospect and value. In this paper, we present a design methodology for a terahertz direct modulator for terahertz direct modulation technology. Experimental results show that the insertion loss is better than 5 dB at 140 GHz, the maximum isolation is 20 dB, and a low VSWR is achieved in the ON/OFF states. We use this modulator to build a terahertz communication system based on the terahertz direct modulation technique, which has a minimal RF architecture, does not require AD/DA for signal acquisition and baseband signal processing, and demonstrates the capability of transmitting and rebroadcasting uncompressed 4K video real-time services over a distance of 40m from the communication link.