Terahertz communication technology can provide abundant frequency resources, strong confidentiality, antijamming capability, communication tracking capability and the ability to achieve highspeed data transmissions and can serve as an important technical method for high-speed communications in the future. Among these terahertz communication technologies, terahertz direct modulation technology is a key means to achieve low system complexity and power consumption. In this paper, a review and outlook of terahertz direct modulation technology are proposed from the aspects of high-electron-mobility-transistor-based terahertz direct modulation, parallelswitch terahertz direct modulation, diode-based terahertz direct modulation, quantum cascade laser-based terahertz direct modulation and new-material-based terahertz direct modulation. We hope through this paper that more readers can gain knowledge about the development and challenges of terahertz direct modulation technology for high-speed communication systems, thus promoting the development of high-speed terahertz communication technology based on direct modulation.